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Certified Histocompatibility Associate (CHA): Practical Preparation Guide Study Guide
Practise specimen acceptance, HLA assays, immune mechanisms, laboratory calculations and safe reporting with 60 focused CHA concepts, 20 questions and 20 recall cards.
By CertMedbry · Updated 31 October 2026
Learn to identify unsuitable specimens, distinguish HLA typing from antibody testing, evaluate assay controls, investigate discordant findings and communicate defensible results. Worked examples connect laboratory and immune mechanisms with decisions a CHA candidate should understand. These 60 concepts selectively cover all five official domains; the shared ACHI CHA/CHT Exam Content Outline defines the full examination scope.
Try 20 free practice questionsWhat is the exam format?
Check the question types, timing and outline before planning your revision.
Which credential and certifying organization does this guide cover?
The guide covers the Certified Histocompatibility Associate (CHA) examination awarded by the American College of Histocompatibility and Immunogenetics (ACHI). The organization was called the American Board of Histocompatibility and Immunogenetics (ABHI) until October 2020. CHT and CHS are separately named credentials with different eligibility requirements; sharing a handbook or outline does not make these credentials interchangeable.
ACHI Examination Candidate Handbook — January 2026 · ACHI CHA, CHS, & CHT Examinations
How many questions are scored, and which item styles are verified?
The January 2026 handbook specifies 150 scored multiple-choice items plus 20 unscored pretest items, giving 170 items altogether. Pretest items are interspersed and do not affect pass/fail status. The handbook describes four answer choices and equal scoring weight for scored items. This guide's 20 original four-option questions are independent learning drills.
How much time is available?
Testing time is 3.5 hours. The handbook describes an additional half hour for instructions and related activities, making a four-hour appointment. Tutorial time does not count toward the examination time or score. Its navigation instructions allow candidates to revisit, bookmark and change answers during the testing period.
How is the examination delivered, and is it adaptive?
The handbook describes computer delivery at Pearson VUE test centers and an OnVUE remote-proctoring option for CHA, CHT and CHS. It presents one question at a time and permits backward navigation and review. It does not identify the examination as computer adaptive; an adaptive format should therefore not be assumed. Candidates using OnVUE must meet its current technical and testing-environment requirements.
What are the official domain allocations?
The shared outline allocates 18 of 150 scored items to Pre-Analytical Systems, 37 to Analytical Systems, 26 to Post-Analytical Systems, 36 to Quality Systems and 33 to General Theory and Applications. Calculated percentages are 12%, 24.67%, 17.33%, 24% and 22%, respectively. This guide uses approximately those proportions: 7, 15, 10, 15 and 13 concepts.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
Which handbook and blueprint dates apply?
The handbook states that its latest update is January 2026. ACHI's Resource Hub links the outline as the CHA/CHT Exam Content Outline, establishing its applicability to CHA although the PDF heading names CHT. The filename contains 20250321, but the outline prints no effective date. Treat that marker as a revision identifier rather than a verified implementation date.
ACHI Resource Hub · ACHI Examination Candidate Handbook — January 2026 · ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
How does CHA eligibility differ from the other credentials?
The January 2026 CHA requirements include 24 hours of qualifying college science coursework, including at least six semester hours each of chemistry and biology, and two years of relevant full-time histocompatibility laboratory experience completed within the specified five-year period. CHT and CHS have separate degree and experience requirements. The handbook also describes special sponsorship for certain applicants whose laboratory experience does not follow the usual accreditation route.
Which scheduling details require clarification?
The handbook's application table lists May and November testing windows, while nearby appointment prose also mentions September. Because these statements differ, use ACHI's current application instructions and the appointment dates actually offered for the intended administration. The handbook describes United States test centers and remote delivery; it does not establish unrestricted availability in every jurisdiction.
Is there a published universal passing percentage?
The handbook explains Angoff standard setting and statistical equating but does not provide a universal passing percentage for use here. Regular score reports include pass/fail status, the raw score and the score needed to pass that examination. These learning questions do not predict certification results. Use the complete official outline to identify additional study needs beyond this selective guide.
ACHI Examination Candidate Handbook — January 2026 · ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
This independent guide prepares for ACHI's Certified Histocompatibility Associate credential. CHA, CHT and CHS are separate certifications. ACHI's Resource Hub explicitly assigns the shared CHA/CHT outline to CHA despite the PDF's CHT heading.
Outline used: Verified against ACHI's Examination Candidate Handbook, latest update January 2026, and the current Resource Hub's shared CHA/CHT Exam Content Outline. The outline filename contains revision marker 20250321; its page does not state an effective date and its heading names Certified Histocompatibility Technologist. Its application to CHA is evidenced by ACHI's shared CHA/CHT link. The assigned CHA name and slug are retained.
60 key concepts: understand, apply and avoid the traps
Work through the questions by exam domain. Explain the answer in your own words, try the example, then check the common pitfall.
Pre-Analytical Systems12% (18/150 items; calculated)
1.How should a specimen identity mismatch be handled?
Specimen identity connects the material tested to the person whose result will be reported. Compare the specimen label with the request and accession record before processing, using the identifiers required by the laboratory. A mismatch creates uncertainty that analytical controls cannot resolve. Hold the specimen outside the routine testing pathway, document the discrepancy and use the defined identification-resolution or recollection process. Urgency changes communication needs, but it does not establish identity.
Apply it
A tube carries the expected name but a different medical record number from the request. Keep it segregated, notify the appropriate staff and document the discrepancy before deciding whether an authorized correction or recollection is possible.
Common pitfall: Relabeling from the request assumes that the request is correct and the tube belongs to that patient. A plausible name match does not prove identity.
2.How does aliquot labeling preserve specimen traceability?
An aliquot must remain linked to its parent specimen throughout preparation, testing and storage. A unique accession or derivative identifier, together with recorded preparation details, allows staff to reconstruct that relationship. This matters when several samples are processed together or a stored aliquot is tested later. Traceability also includes location and disposition, so that a result investigation can identify which physical material was used rather than merely which patient was intended.
Apply it
Two serum aliquots are prepared for separate antibody runs. Label each immediately with its traceable identifier, record its parent specimen and storage location, and verify the identity again when loading the later run.
Common pitfall: Rack position and a handwritten patient initial are weak substitutes for a traceable identifier. Moving tubes can destroy an otherwise apparently obvious relationship.
3.Which specimen characteristics determine suitability for an assay?
Suitability depends on the analyte and the method, not simply on whether a tube contains patient material. Antibody testing needs an accepted specimen containing the relevant antibodies; cellular crossmatching needs suitable target cells; molecular typing needs usable nucleic acid. Check the required specimen type, amount, collection conditions and condition on arrival against the assay's acceptance criteria. A specimen accepted for one method does not automatically satisfy another method's requirements.
Apply it
A laboratory receives only an extracted DNA sample for a request that includes HLA typing and a donor-cell flow crossmatch. It can assess the DNA for typing, but must obtain the additional materials required for crossmatching.
Common pitfall: Treating every blood-derived specimen as interchangeable ignores which analyte or cell population the assay measures. Adequate volume alone does not establish fitness.
WHO LQMS Module 5 — Sample Management · Versiti — Crossmatch (Flow Cytometry) Against Recipient · One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays
4.Why must specimen storage conditions follow the intended test?
Storage must preserve the property that the assay will measure, such as antibody reactivity, usable DNA or suitable cells. Record relevant collection, receipt, processing and storage information so that acceptability can be assessed against established limits. An excursion requires evaluation of its duration and possible effect on the intended assay. Moving the specimen into acceptable storage afterward does not reverse an earlier loss of integrity or erase the need for documentation.
Apply it
A stored specimen is discovered outside its assigned temperature range. Record the known exposure period, identify the planned assays and assess their stability requirements before returning the specimen to the testing queue or requesting replacement.
Common pitfall: A specimen's normal appearance does not establish analyte stability. Conversely, an excursion should be evaluated against relevant limits rather than automatically assigned an invented universal rejection rule.
5.How should reagent inventory be rotated and monitored?
Inventory records should identify material, quantity, lot, expiration and storage location, allowing staff to maintain supplies without using unsuitable stock. Rotate usable materials by the earliest applicable expiration rather than purchase date alone. Separate expired, damaged or otherwise unacceptable supplies from available inventory. Monitoring consumption and delivery times supports timely ordering, while receipt checks establish that newly delivered materials match the order and meet the laboratory's acceptance conditions before use.
Apply it
A newly delivered kit expires before an older kit already in storage. After receipt acceptance, place the earlier-expiring usable kit first in the working sequence and update inventory so both quantities and dates remain visible.
Common pitfall: First received does not always mean first to expire. Stock rotation cannot make an expired or improperly stored reagent suitable for patient testing.
6.What must be controlled when a reagent is reconstituted?
Reconstitution changes a supplied material into its working form, so the specified diluent, volume and handling conditions determine the resulting concentration and fitness. Document preparation and label the reagent with its identity, lot linkage, preparation information and applicable expiration or in-use limit. Distinguish an unopened manufacturer's expiration from a shorter limit after opening or preparation. Incorrect volume can create a systematic concentration error even when the reagent appears completely dissolved.
Apply it
A vial requires 2.0 mL of its specified diluent, but 2.5 mL is added. Identify and segregate the preparation, document the error and follow the established disposition process instead of treating complete dissolution as acceptance.
Common pitfall: Copying the unopened expiration onto every prepared reagent can overstate its usable life. Appearance and solubility do not verify concentration or stability after reconstitution.
WHO LQMS Module 4 — Purchasing and Inventory · WHO LQMS Module 16 — Documents and Records
7.How should assay resolution and sensitivity guide method selection?
Select a method by the question it must answer. Typing resolution describes how specifically genetic alternatives can be distinguished; antibody sensitivity concerns the ability to detect reactivity under defined conditions. These are different performance properties. Review the required information, available specimen, method limitations and suitable equipment before choosing an assay. A technically successful method can still be unsuitable if its reportable detail or detection capability cannot address the intended clinical or laboratory purpose.
Apply it
A request requires distinction between two allele alternatives that share the same result in the available typing panel. Choose a validated method that can interrogate the distinguishing sequence rather than repeating a panel incapable of resolving it.
Common pitfall: Calling a method more sensitive does not establish higher typing resolution. Performance claims must refer to the analyte, intended use and alternatives the method actually distinguishes.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist · One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays
Analytical Systems24.67% (37/150 items; calculated)
8.What does a class I single-antigen bead assay detect?
A class I single-antigen bead assay exposes serum antibodies to beads carrying defined HLA class I antigen targets. Reactivity is assigned to the antigen associated with each bead population, allowing antibody specificity patterns to be investigated. The assay measures antibody binding under its test conditions; it does not determine the patient's HLA genotype. Interpretation requires acceptable controls and consideration of the complete pattern, because an isolated signal is not automatically a donor-specific compatibility conclusion.
Apply it
A serum reacts with beads bearing several defined class I antigens. Compare that specificity pattern with the donor's established HLA typing when assessing possible donor specificity, while retaining the assay's controls and interpretation limits.
Common pitfall: A reactive antigen bead identifies an antibody target in the assay. It does not show that the recipient inherited that antigen or establish a transplant decision alone.
9.How does anti-human IgG reveal bead-bound antibody?
In an IgG antibody assay, patient antibody first binds an antigen target. A labeled anti-human IgG detection reagent then binds the human IgG associated with that target, producing the measured signal. Target specificity and immunoglobulin detection therefore come from different assay components. Acceptable performance requires both antigen binding and detection steps to function. Weak signal can reflect little bound IgG, but it can also arise from a preparation, detection or interference problem.
Apply it
A bead run shows unexpectedly weak patient signals and an unacceptable positive control. Review reagent preparation, detection reagent addition and instrument performance before interpreting the patient specimens as lacking detectable HLA IgG antibodies.
Common pitfall: The detection reagent recognizes human IgG; it does not independently identify which HLA antigen was recognized. The bead target supplies that specificity information.
10.Why must antibody specificity be distinguished from donor specificity?
An antibody specificity describes the antigen target recognized in the assay. Donor specificity additionally requires evidence that the donor expresses a corresponding target, based on adequate donor typing and the interpretation framework. A patient may have HLA antibodies without those antibodies being directed against the particular donor. Conversely, incomplete donor typing can leave a relevant comparison unresolved. Keep the antibody finding, donor information and compatibility interpretation distinguishable in the analytical review.
Apply it
A recipient has reproducible reactivity to an antigen absent from the donor's completed typing. Record the antibody finding accurately, but do not label it donor-specific merely because it is an HLA antibody in a transplant candidate.
Common pitfall: A positive antibody assay is not synonymous with a positive donor-specific finding. Missing donor information also must not be treated as proof of antigen absence.
One Lambda LABScreen Single Antigen HLA Class I — Combi · Versiti — Crossmatch (Flow Cytometry) Against Recipient
11.Why is bead fluorescence not an absolute antibody concentration?
Fluorescence reflects the assay's measured binding signal, influenced by the target presentation, detection system and specimen conditions. It should not automatically be converted into an absolute antibody concentration or a universal clinical risk category. Complement-related interference can even suppress a signal in a strongly reactive specimen. Interpret signal strength with control performance, specificity patterns and validated laboratory criteria. Comparisons are most defensible when the relevant method and specimen treatment conditions are comparable.
Apply it
Two reports from different assay conditions show different fluorescence values for the same target. Compare methods, controls and specimen treatments before describing the difference as a biological rise or fall in antibody amount.
Common pitfall: A larger signal can support stronger measured reactivity under comparable conditions, but it does not justify a universal concentration conversion or transplant cutoff.
One Lambda LABScreen Single Antigen HLA Class I — Combi · HLA Antibody Specification Using Single-antigen Beads—A Technical Solution for the Prozone Effect
12.How do sequence-specific primers discriminate HLA sequences?
Sequence-specific primer typing uses amplification reactions designed to occur when specimen DNA matches the primer requirements. Primers bind complementary sequences and provide starting points for DNA polymerase, which extends the DNA. A probe instead identifies a complementary target by hybridization; it does not supply polymerase's synthesis activity. An SSP panel's positive and negative reaction pattern is compared with its interpretation information to assign supported typing alternatives. This assesses DNA rather than circulating antibody. Specificity depends on primer design and reaction conditions, so valid controls and the complete pattern are necessary before assigning a result.
Apply it
A DNA sample produces a reproducible pattern across an SSP tray with acceptable controls. Compare the entire pattern with the correct panel interpretation information, retaining any unresolved allele alternatives allowed by that pattern. If unrelated reactions and their required controls all fail, investigate shared amplification components before excluding every target.
Common pitfall: One positive SSP well rarely supplies a complete HLA assignment. Polymerase synthesizes DNA; sequence discrimination comes from the designed assay and its interpretation rules.
One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays · 2025_ashi_standards.pdf
13.What does an SSP internal amplification control establish?
An internal amplification control provides evidence that the reaction can amplify a designated control target. When the expected control is absent in a reaction whose interpretation requires it, lack of target amplification cannot confidently be called a true negative. Possible explanations include unsuitable DNA, inhibition, missing components or reaction failure. Apply the assay's control rules to the affected reactions before interpreting the typing pattern, rather than converting every absent target band into allele absence.
Apply it
A required control product and the HLA-specific product are both absent from a reaction. Classify that reaction according to the assay's invalidity rules and investigate amplification performance before using it to exclude an allele.
Common pitfall: No target product means target absence only when the reaction is interpretable. A failed required control removes the basis for that negative conclusion.
One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays
14.How should an SSP pattern with multiple compatible typings be reported?
A valid amplification pattern can remain compatible with more than one allele assignment because the tested sequences do not distinguish every alternative. This is limited resolution, not automatically a failed assay. Identify the alternatives supported by the panel and determine whether the requested use requires further discrimination. Additional testing should examine information that separates the remaining possibilities. Report the supported resolution and ambiguity rather than selecting a familiar allele without distinguishing evidence.
Apply it
Two allele pairs fit every valid reaction in a panel. Preserve both possibilities in the interpretation and, if the request requires distinction, select follow-up testing that targets their differing sequence rather than repeating identical reactions.
Common pitfall: Repeating an inherently nondiscriminating panel may reproduce the ambiguity perfectly. Reproducibility confirms the pattern but does not create information the panel never measured.
One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays · HLA Nomenclature — Naming Alleles
15.Why are ABO grouping and HLA typing separate measurements?
ABO grouping evaluates the ABO blood group system, while HLA typing identifies histocompatibility markers. The methods, targets and interpretation rules differ. Forward ABO grouping examines red-cell antigen reactivity; molecular HLA typing examines selected DNA sequences. Agreement in one system does not establish agreement in the other. When both are requested, ensure the correct specimen, reagents, controls and report fields are used for each rather than allowing one compatibility result to substitute for the other.
Apply it
A donor and recipient share an ABO group, but their HLA typing has not been compared. Complete the requested HLA assessment separately; the ABO agreement supplies no evidence that their HLA assignments also match.
Common pitfall: The word compatibility can conceal different biological questions. An ABO match does not establish an HLA match or replace antibody and crossmatch assessment.
Canadian Blood Services — Pre-transfusion Testing · One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays
16.Which materials are combined in a recipient-directed flow crossmatch?
A recipient-directed flow crossmatch tests recipient serum against donor lymphocytes to assess antibody binding to the donor cells under the assay conditions. The biological direction matters: recipient antibodies are being evaluated against donor targets. Accurate donor-cell and recipient-serum identities are therefore essential. A result describes the tested pairing and specimen, not every possible donor or every antibody the recipient might possess. Interpretation also depends on the method's controls and cell analysis.
Apply it
A donor-cell suspension and two recipient sera arrive together. Verify each pairing before setup and record which serum was incubated with which donor cells, so the resulting crossmatch can be attributed to the correct combination.
Common pitfall: Reversing the serum and cell roles changes the biological question. A correctly functioning instrument cannot detect that the wrong donor-recipient pairing was assembled.
17.How do cell-based crossmatching and single-antigen testing differ?
Single-antigen testing examines binding to defined HLA targets on beads, whereas a flow crossmatch examines binding to the donor cells used in that test. These substrates and questions differ, so complete agreement is not guaranteed. Antibody specificity results can help interpret whether a donor expresses a recognized target, while the crossmatch supplies a cell-based observation. Evaluate each assay's validity and limitations before combining the findings into a compatibility assessment.
Apply it
A recipient has a bead antibody finding but an unexpected donor-cell crossmatch result. Confirm donor typing, specimen identity and both assays' controls, then investigate the differing substrates instead of assuming either method must always overrule the other.
Common pitfall: Calling both assays antibody tests does not make their outputs interchangeable. A bead specificity pattern and binding to donor cells require a connected interpretation.
One Lambda LABScreen Single Antigen HLA Class I — Combi · Versiti — Crossmatch (Flow Cytometry) Against Recipient
18.What does forward ABO grouping measure?
Forward ABO grouping tests the patient's red cells with defined anti-A and anti-B reagents to determine whether the corresponding antigens are detected. The reaction belongs to the red-cell antigen side of ABO assessment. Record the actual reaction pattern before assigning a group and compare it with the reverse grouping when that comparison applies. A weak, mixed or unexpected reaction needs investigation rather than being rounded into the expected historical group.
Apply it
Red cells react with anti-A and do not react with anti-B. Record that forward pattern, then compare it with the plasma reaction pattern before finalizing an ABO assignment for this specimen.
Common pitfall: Forward grouping does not test the patient's plasma for anti-A or anti-B. Confusing the tested material reverses the interpretation of the reaction.
19.What does reverse ABO grouping measure?
Reverse ABO grouping tests the patient's plasma against reagent red cells of known ABO characteristics. It evaluates expected plasma antibody reactivity and provides a complementary check on forward grouping. For a typical adult group A pattern, plasma reacts with group B reagent cells and not group A reagent cells. Interpret the observed reactions in context because expected antibody patterns are not equally reliable in every patient population or clinical situation.
Apply it
An adult's red cells show a group A forward pattern. Plasma reacts with reagent B cells but not reagent A cells. Recognize that the two directions support the same ABO assignment under the stated conditions.
Common pitfall: A reaction with reagent B cells indicates plasma reactivity against those cells; it does not mean the patient's own red cells carry B antigen.
20.When does an ABO discrepancy prevent a routine group assignment?
When forward and reverse ABO findings do not support the expected common interpretation, the discrepancy must be recognized and resolved before a routine final assignment is made. First verify identity, setup, reagents and recorded reactions, then investigate the particular unexpected finding. Historical information helps frame the investigation but does not erase current discordance. The key decision is whether the evidence supports a defensible group, rather than whether one portion looks familiar.
Apply it
An adult's forward grouping resembles group A, but the plasma reacts with neither reagent A nor reagent B cells. Document the missing expected reverse reaction and investigate before issuing a routine concordant group A result.
Common pitfall: Choosing the forward result simply because it appears clearer bypasses the purpose of the reverse check. A history match does not resolve current discordance.
21.How do informative genetic markers support chimerism testing?
Post-transplant chimerism testing distinguishes donor-derived from recipient-derived genetic material using markers that differentiate the two individuals. A marker shared identically by donor and recipient cannot independently identify the source of a measured signal. Establishing informative differences supports the later estimate of donor and recipient contributions. The result concerns the tested specimen and analytical method; it is not itself a direct measurement of graft function or a complete assessment of disease status.
Apply it
Baseline donor and recipient samples have identical results at one marker but differ at several others. Use informative markers supported by the assay for source discrimination rather than treating the shared marker as evidence of recipient persistence.
Common pitfall: A detectable genetic marker is not automatically informative. It must distinguish donor from recipient within the method's validated interpretation framework to support source attribution.
22.How should serial chimerism percentages be compared?
A chimerism percentage estimates a genetic contribution within the specimen or cell fraction actually tested. Compare serial values only after checking specimen identity, cell fraction, method and report conventions. A whole-blood measurement and a selected cell fraction can describe different mixtures, so their percentages need not match. Trends may be clinically useful, but the laboratory must preserve what was measured and avoid converting a percentage alone into an unsupported diagnosis or treatment instruction.
Apply it
A whole-blood result is followed by a result from a selected cell fraction. Before describing the numerical difference as a temporal change, identify the differing specimen populations and seek a comparable series for trend assessment.
Common pitfall: One hundred percent donor signal in the tested material is not a universal claim about every tissue or proof that every post-transplant complication is absent.
Post-Analytical Systems17.33% (26/150 items; calculated)
23.How can complement interference conceal HLA antibody reactivity?
Complement-related interference can suppress detection in a single-antigen bead assay, producing unexpectedly weak or negative signals despite antibody being present. Original research describes this prozone-related problem and approaches to revealing concealed reactivity. A stronger signal after an appropriate validated dilution or treatment is a clue to interference, not evidence that dilution created antibody. Investigate the pattern with suitable controls and document the specimen treatment so the final interpretation reflects the conditions that produced it.
Apply it
An undiluted specimen shows little reactivity, but a validated interference investigation produces stronger signals after dilution. Review the controls and specificity pattern, and document the interference assessment before interpreting the original low signal as antibody absence.
Common pitfall: Dilution does not guarantee a weaker measured signal when interference is present. An unexpected increase should trigger evaluation rather than automatic dismissal as impossible.
HLA Antibody Specification Using Single-antigen Beads—A Technical Solution for the Prozone Effect
24.How should a reproducible but ambiguous typing result be resolved?
Separate technical validity from interpretive uniqueness. An assay can perform correctly while leaving several genetic alternatives unresolved. Confirm that the ambiguity is compatible with the method's capabilities, then identify which additional sequence information would distinguish the alternatives. Further testing is useful when it supplies that information and the intended use requires it. Until resolved, preserve the ambiguity and reportable resolution instead of substituting population frequency, familiarity or expectation for direct discrimination.
Apply it
Repeated valid testing supports the same two allele alternatives. Review their distinguishing sequence and select an appropriate additional method if needed, while retaining the unresolved alternatives in any interim communication about the result.
Common pitfall: The more common allele may be a tempting guess, but prevalence does not establish this individual's genotype when the observed test pattern supports multiple alternatives.
One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays · HLA Nomenclature — Naming Alleles
25.What should be investigated when current typing conflicts with history?
A conflict with a historical result is a discrepancy requiring investigation, not a reason to choose whichever report is newer. Verify patient and specimen identity, the original records, typing resolution, method limitations and relevant clinical context. Distinguish a true incompatible assignment from a more detailed result that merely refines an earlier one. Document the comparison and resolution so that future users can understand whether the difference reflected identity, transcription, methodology or interpretation.
Apply it
A current allele-level report appears different from an older antigen-level entry. Compare the naming levels and original records first; determine whether the new result refines the old assignment or actually contradicts it before initiating corrective reporting.
Common pitfall: Different text is not always different biology. Conversely, repeating a mislabeled specimen can reproduce a discrepancy without resolving the underlying identity problem at all.
WHO LQMS Module 14 — Occurrence Management · HLA Nomenclature — Naming Alleles
26.Why should historical antibody findings remain visible?
A current antibody assay describes reactivity detected in the current specimen under its test conditions. A previously documented finding remains part of the patient's laboratory history even if it is not detected now. Compare dates, specimen treatments, methods and specificity information before describing a change. Preserve the distinction between historical detection and current detection; neither should be silently overwritten. This allows authorized reviewers to interpret the complete evidence rather than a misleadingly simplified record.
Apply it
A current bead assay does not reproduce a previously recorded specificity. Verify the historical report and compare testing conditions, then communicate the current finding while retaining the earlier result as a separately dated observation.
Common pitfall: Deleting a historical positive because today's assay is negative destroys relevant evidence. History should also not be copied into the current result as if newly detected.
WHO LQMS Module 16 — Documents and Records · One Lambda LABScreen Single Antigen HLA Class I — Combi
27.How should results from different methods be compared?
Before comparing numbers or classifications, identify what each method measures, its specimen conditions, units and interpretation rules. A bead-binding signal, a cell-based crossmatch observation and an allele assignment are different outputs. Even similar-looking numerical results may not be directly interchangeable across methods. Describe agreement or disagreement at the appropriate biological level, and investigate specific discrepancies rather than forcing all methods into one scale. Preserve method details when communicating a comparison.
Apply it
A clinician asks whether a bead fluorescence value equals a particular crossmatch result. Explain the different substrates and outputs, then review the actual antibody specificity, donor information and crossmatch findings together rather than offering a numerical conversion.
Common pitfall: Two numbers can look comparable while measuring different properties. A convenient conversion without a validated relationship creates precision that the evidence does not support.
One Lambda LABScreen Single Antigen HLA Class I — Combi · Versiti — Crossmatch (Flow Cytometry) Against Recipient
28.When is an uninterpretable result different from a negative result?
A negative result requires evidence that the assay was capable of detecting its target under acceptable conditions. If a required control fails, specimen suitability is unresolved or a critical reaction is invalid, the result may be uninterpretable rather than negative. State the limitation precisely and identify what is needed to obtain a valid result. This distinction protects users from treating missing reliable information as affirmative evidence that an antibody, sequence or other analyte is absent.
Apply it
An SSP reaction lacks both the expected target product and its required internal control. Communicate that the reaction cannot support allele exclusion and arrange the appropriate investigation or repeat, rather than releasing a simple negative.
Common pitfall: Blank, failed and negative are different result states. Reporting failure as negative conceals uncertainty and can make an incomplete investigation look like definitive testing.
One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays · WHO LQMS Module 7 — Quantitative Quality Control
29.What details make a typing report interpretable?
A useful typing report identifies the patient and specimen, the tested loci or targets, the supported result and any resolution or ambiguity that affects interpretation. Method and relevant limitations help users understand what the assignment does and does not distinguish. Review nomenclature and transcription against the analytical record before release. Reporting additional allele fields without evidence exaggerates resolution, while omitting a meaningful expression suffix or unresolved alternative can change how the result is understood.
Apply it
The analytical record supports two possible allele assignments, but the draft report contains only one. Correct the report before authorization so it preserves the supported alternatives and accurately describes the achieved typing resolution.
Common pitfall: A more specific-looking report is not necessarily a better report. Unsupported precision can mislead downstream matching just as an incorrect allele designation can.
HLA Nomenclature — Naming Alleles · WHO Laboratory Quality Stepwise Implementation Checklist
30.How should an urgent laboratory result be communicated?
Urgent communication must deliver the reviewed result to an authorized person who can act on it, within the laboratory's defined process. Confirm the patient, result and any limitations, then document when the communication occurred, who provided it and who received it. Use the required confirmation process to reduce misunderstanding. Sending a message is not equivalent to establishing receipt, and verbal communication does not eliminate the need for an accurate authorized report.
Apply it
A reviewed result meets the laboratory's urgent-notification criteria near shift change. Contact the designated authorized recipient, confirm the message as required and record the communication details so responsibility does not disappear between shifts.
Common pitfall: An unacknowledged message may not close the communication loop. Urgency also does not justify omitting identity checks, result review or important interpretive limitations.
31.How should a released report be amended?
An amendment must make the corrected information clear while preserving the original report and the history of the change. Document the reason, authorization and timing, and notify appropriate recipients when the change affects interpretation or action. The record should show which result was available at each point. Quietly overwriting the original conceals the information that users may already have relied upon and prevents a complete investigation of the reporting error.
Apply it
A released HLA report contains a transcription error despite a correct analytical worksheet. Issue an authorized amended report, preserve the original and document notification to the clinical recipient rather than silently replacing the displayed allele.
Common pitfall: Correcting the database alone does not establish that earlier recipients received the correction. An amendment needs traceable content, authorization and communication appropriate to its impact.
WHO Laboratory Quality Stepwise Implementation Checklist · WHO LQMS Module 16 — Documents and Records
32.How should a result requester's authority be verified?
Laboratory results are confidential patient information, so release must follow an established authorization process. Verify the requester's identity and entitlement to receive the particular information, using a trusted route rather than relying only on a claimed role. Confirm the patient and use an approved communication channel. Access controls and disclosure records support accountability. Urgency or familiarity with a patient's name does not by itself establish that the requester is authorized to receive results.
Apply it
An unfamiliar caller requests an urgent donor-recipient report and gives a clinical title. Verify the caller through the laboratory's established contact process and confirm authorization before disclosing results through the approved communication channel.
Common pitfall: Knowing a patient's name or accession number is not proof of authorization. A helpful response can still breach confidentiality if identity and access are unverified.
Quality Systems24% (36/150 items; calculated)
33.Which protection is needed when a procedure can splash?
Choose personal protective equipment according to the exposure route created by the task. Gloves protect hands but do not protect eyes or facial mucous membranes from splashes. Procedures with splash potential therefore require the appropriate eye or face protection and other protective clothing specified by the laboratory's risk assessment. Engineering controls and careful technique complement personal protection. Evaluate the actual procedure rather than assuming a familiar specimen or a short task cannot create exposure.
Apply it
A technologist prepares to open and transfer several specimens using a process with splash potential. Before starting, use the designated eye or face protection and protective clothing in addition to gloves, and check the appropriate workspace controls.
Common pitfall: Gloves address only part of the exposure pathway. A procedure can remain hazardous to eyes and face even when hands are completely covered.
34.Why must gloves be changed between contaminated and clean tasks?
Gloves can carry contamination from specimens and work surfaces to keyboards, phones and clean supplies. Their protective value depends on correct task boundaries, removal and hand hygiene rather than continuous wear. Remove contaminated gloves before clean activities and replace them when required for the next hazardous task. Hand hygiene remains necessary after removal because gloves can be compromised and removal can transfer contamination. Clean-area discipline protects both personnel and the integrity of laboratory work.
Apply it
After handling specimens, a worker needs to use a shared office keyboard. Remove the gloves and perform hand hygiene before the clean task, then use fresh gloves if returning to work that requires them.
Common pitfall: Wearing the same gloves everywhere spreads contamination while creating a false sense of protection. Gloves do not replace hand hygiene or clean-area separation.
35.How should a laboratory exposure incident be managed?
Immediately flush an exposed eye or other mucous membrane with water. Wash exposed skin with soap and water. Promptly report an exposure incident and obtain medical evaluation through the designated occupational-exposure process; do not postpone these actions while identifying every detail of the source material. OSHA 29 CFR 1910.1030(d)(2)(vi) specifies washing skin or flushing mucous membranes immediately or as soon as feasible, and paragraph (f)(3) requires an immediately available confidential medical evaluation and follow-up after a reported exposure incident. These provisions do not prescribe one universal irrigation duration. See the [OSHA bloodborne-pathogens standard](https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1030/).
Apply it
A specimen splashes into a worker's eye. Begin flushing the eye with water immediately, report the incident promptly and obtain the designated medical evaluation. Document the exposure route and circumstances for follow-up and investigation. If material instead contacts skin, wash that area with soap and water.
Common pitfall: Do not put soap or surface disinfectant into an exposed eye. Reporting and medical evaluation follow immediate flushing; paperwork must not delay the first response.
WHO LQMS Module 2 — Facilities and Safety · WHO LQMS Module 14 — Occurrence Management
36.Which concrete storage and proficiency-testing obligations apply under CLIA and ASHI?
For laboratories subject to CLIA histocompatibility requirements, 42 CFR 493.1278(a) requires continuous monitoring of donor/recipient specimen and reagent storage temperatures, an alert system notifying personnel when limits are exceeded, written specimen-specific storage and retention procedures, retrievable specimens and an emergency plan for alternate storage. For ASHI-accredited laboratories, standards C.1.2–C.1.4 require proficiency-testing samples to enter the regular workload, receive testing comparable to routine clinical specimens without extra testing, and rotate among testing personnel. Laboratories must not discuss PT results with other laboratories before the reporting deadline or send PT samples or results to another laboratory for analysis. The communication restriction includes laboratories sharing a director.
Apply it
A freezer alarm activates overnight: the response must reach responsible personnel and support transfer to planned alternate storage when needed. Separately, an uncertain PT sample must be handled within routine testing rules; sending it to a reference laboratory or comparing answers with another laboratory before the deadline is prohibited.
Common pitfall: A daily temperature entry alone does not satisfy continuous monitoring and alert requirements. Routine patient referral arrangements do not authorize PT referral.
Federal Register :: Request Access · 2025_ashi_standards.pdf
37.What records allow a patient result to be reconstructed?
A reconstructable record links the patient and specimen to the method, relevant reagent lots, instrument or run, controls, observations, interpretation and authorization. Preserve original entries and trace subsequent corrections rather than erasing the sequence of events. These records support investigations and show whether the conditions for release were met. A final result alone cannot explain how it was obtained or identify which other specimens might be affected by a shared reagent or instrument problem.
Apply it
A reagent lot is later found unsuitable. Use the recorded lot-to-run and run-to-specimen links to identify potentially affected results, then review their analytical and release records instead of searching only by report date.
Common pitfall: Recording that testing was completed is not enough to reconstruct it. Missing lot or control links can prevent a focused investigation of affected patients.
WHO LQMS Module 16 — Documents and Records · WHO LQMS Module 3 — Equipment
38.How does document control prevent inconsistent testing?
Document control ensures that staff use authorized, current instructions and that changes are reviewed, approved and traceable. Procedures and job aids should have identifiable versions and controlled availability, with obsolete working copies removed from use. Records differ from instructions: records preserve what actually happened and must remain legible and retrievable. Updating a master procedure without updating the bench copy leaves a practical gap that can produce inconsistent testing despite an apparently current document system.
Apply it
A revised assay instruction changes a preparation step, but an older laminated aid remains beside the instrument. Remove the obsolete working aid, replace it with the approved version and ensure affected staff understand the change.
Common pitfall: An unofficial handwritten shortcut can become the real procedure if staff rely on it. Helpful job aids still require appropriate review and control.
39.When should a failed control stop patient-result release?
Quality controls test whether the analytical system meets its acceptance requirements. When a required control fails, hold affected patient results and investigate the failure before release. Consider the failure's scope, including which runs or results may be affected, and document the corrective steps and evidence of restored performance. Repeating only until a control happens to pass does not explain the original failure. Release depends on a defensible assessment, not merely a later acceptable number.
Apply it
The run's required positive control is outside its established limits while patient results appear plausible. Hold the affected results, investigate the control failure and establish acceptable system performance before authorizing any results from that run.
Common pitfall: Plausible patient findings cannot substitute for required control acceptance. Selectively keeping a favorable repeat can hide an unresolved analytical problem rather than correct it.
40.How do accuracy and precision reveal different errors?
Precision describes agreement among repeated measurements; accuracy describes closeness to an accepted reference value. A method can produce tightly clustered results that are consistently displaced from the reference, indicating good precision with bias. Widely scattered results suggest poor precision even if their average is near the reference. Examine both properties when evaluating performance because repetition alone cannot establish correctness. The pattern helps direct investigation toward systematic displacement, random variation or a combination of problems.
Apply it
A control with an accepted value of 100 repeatedly measures 119, 120 and 121. Recognize a tight but displaced cluster, investigate systematic bias and avoid describing the method as accurate merely because the repeats agree.
Common pitfall: Reproducibility does not prove truth. A consistently incorrect calibration or preparation can generate very precise measurements that remain unsuitable for the intended use.
41.Why should control trends be reviewed before outright failure?
Control monitoring evaluates patterns over time as well as individual observations. A persistent shift or gradual drift can indicate a developing change in reagents, calibration, equipment or technique even while individual results remain within broad limits. Apply the laboratory's established review rules and investigate meaningful patterns. Trend review is preventive: it can identify deteriorating performance before a major failure. An isolated acceptable value should not automatically erase a sustained pattern requiring attention.
Apply it
A control moves progressively upward over several runs while remaining inside its broad acceptance interval. Review the trend against established rules and examine recent reagent, maintenance and calibration changes before assuming the system is stable.
Common pitfall: Inside the limits does not always mean unchanged performance. Trend interpretation must follow established rules rather than inventing rejection criteria after seeing the data.
42.How do calibration and maintenance serve different purposes?
Calibration establishes the relationship between an instrument's response and an appropriate reference, while maintenance keeps equipment in suitable operating condition. Cleaning, replacing parts or servicing a device does not automatically demonstrate accurate measurement afterward. Conversely, a successful calibration does not replace required mechanical or preventive care. Equipment records should distinguish these activities and document the performance checks required before return to patient testing, particularly when service could affect measurement or analytical behavior.
Apply it
A pipettor is repaired after a mechanical fault. Record the repair and complete the required volume-performance assessment before returning it to reagent preparation rather than assuming the new part guarantees correct delivery.
Common pitfall: A service sticker documents an activity, not necessarily acceptable analytical performance. Determine which checks demonstrate that the equipment is fit for its intended use.
43.What establishes that new equipment is ready for testing?
Before routine use, assess whether equipment performs acceptably in its actual laboratory setting and intended application. Confirm installation and operating conditions, complete relevant performance checks and document acceptance against predefined criteria. Manufacturer specifications inform the evaluation but do not replace local evidence that the device functions properly after delivery and setup. Training, operating instructions and maintenance arrangements also need to be in place so acceptable initial performance can be sustained and investigated if it changes.
Apply it
A replacement instrument produces its first control readings after installation. Review the planned acceptance studies and required documentation before authorizing patient use, rather than treating a single successful startup as complete performance evaluation.
Common pitfall: Being new or factory tested does not establish readiness in the receiving laboratory. Acceptance requires evidence relevant to the actual installation and intended work.
44.How does CLIA verification differ from establishing assay performance specifications?
Before reporting patient results, CLIA 42 CFR 493.1253 requires laboratories introducing an unmodified FDA-cleared or approved system to verify manufacturer-comparable accuracy, precision and reportable range, plus suitability of reference intervals for their patient population. For modified cleared systems and applicable systems without FDA clearance or approval, laboratories must establish accuracy, precision, analytical sensitivity, analytical specificity including interfering substances, reportable range, reference intervals and other necessary characteristics, as applicable. Document the studies and establish calibration and control procedures from the verified or established specifications.
Apply it
For a modified chimerism assay, hypothetical criteria require bias within 1 percentage point, SD ≤0.3 percentage point at 10% recipient DNA, and detection in at least 19/20 replicates at 0.5%. Forty comparison mixtures meet the bias criterion; replicates across five days give SD 0.2. Only 18/20 low-level replicates are detected: the sensitivity criterion fails. Investigate before release; these studies also leave specificity/interference, range and other applicable requirements to address.
Common pitfall: These acceptance limits are hypothetical. Passing accuracy and precision cannot compensate for failed sensitivity or omitted required studies.
45.How does root-cause analysis differ from immediate correction?
Immediate correction addresses the discovered problem, such as replacing a mislabeled working reagent or issuing a corrected report. Root-cause analysis asks why the problem occurred and what process conditions allowed it to reach that stage. Corrective action then targets those causes to reduce recurrence. Document the event, affected scope, investigation and action rather than stopping after the visible error is fixed. A useful explanation identifies controllable process factors instead of merely naming the person involved.
Apply it
Repeated transcription errors are corrected individually. Review how results move from the analytical record into the reporting system, identify the failure mechanism and implement a change that addresses that transfer rather than issuing another general reminder.
Common pitfall: Staff error describes an event more often than a root cause. Repeating training without examining workload, interface design or verification steps may leave recurrence unchanged.
46.How is corrective-action effectiveness demonstrated?
An action is not proven effective merely because it was implemented. Define a measure linked to the original failure, review performance over an appropriate interval and determine whether recurrence or impact has decreased. Use a meaningful denominator when workload changes, such as errors per report rather than raw errors alone. Preserve the review and any further action required. This connects occurrence management with quality assurance and process improvement instead of treating completion of paperwork as the endpoint.
Apply it
After introducing a result-verification change, compare transcription errors per 1,000 reports before and after implementation. Review enough subsequent work to judge the change and investigate persistent errors rather than closing the event on the installation date.
Common pitfall: Fewer errors during a much quieter month may not mean improvement. An effectiveness measure should reflect the failure mechanism and account for the work performed.
WHO LQMS Module 14 — Occurrence Management · WHO LQMS Module 7 — Quantitative Quality Control
47.Why should a new reagent lot be assessed before routine use?
A reagent's identity and expiration do not alone establish acceptable performance in the laboratory's assay. Check receipt and storage conditions, maintain lot traceability and perform the required lot-acceptance assessment against predefined criteria. A new lot can introduce a change that becomes apparent only through suitable controls or comparison materials. If performance is unacceptable, segregate the material and investigate before patient use. Documenting the change also helps explain later shifts in control or patient-result patterns.
Apply it
A new detection-reagent lot arrives before the old one is exhausted. Complete the required acceptance assessment while suitable comparison material is available, document the result and release the lot for use only when its criteria are met.
Common pitfall: The same product number does not prove identical observed performance across lots. Conversely, lot comparison needs established criteria rather than an assumption of exact numerical identity.
WHO LQMS Module 4 — Purchasing and Inventory · WHO LQMS Module 7 — Quantitative Quality Control
General Theory and Applications22% (33/150 items; calculated)
48.How do HLA allele fields and expression suffixes preserve distinct information?
An HLA allele designation begins with the locus, followed by colon-separated fields. The first field identifies an allele group, often related to a serological designation. The second distinguishes encoded protein sequences; the third distinguishes synonymous coding substitutions; the fourth distinguishes noncoding sequence differences. Thus HLA-A*02:01 and HLA-A*02:05 share a group but encode different proteins. Names sharing their first two fields can differ genetically while encoding the same protein. Expression suffixes add another property: N denotes null expression, L low cell-surface expression and Q questionable expression. Preserve both supported sequence detail and any suffix when reporting or transferring results.
Apply it
Compare three records: HLA-A*02:01 versus HLA-A*02:05 differs at the protein field; two names differing only at field three describe a synonymous coding distinction. If an imported allele loses its N suffix, correct the transfer because the null-expression annotation has been lost even though the numbered fields match.
Common pitfall: Fields are not decimal places. Additional fields need not change the protein, and low, null and questionable expression are not interchangeable annotations.
49.Why does a secondary immune response differ from a primary response?
A primary adaptive response recruits antigen-specific naive lymphocytes, which must activate, expand and differentiate into effector and memory populations. On subsequent exposure, memory lymphocytes provide a faster response; memory B cells can rapidly become antibody-secreting cells. Secondary antibody responses commonly show greater magnitude and more affinity-matured, class-switched antibody, although memory B cells are not exclusively class-switched. Long-lived plasma cells and memory cells serve different roles: plasma cells secrete antibody, while memory cells support recall. [Primary research on naive and memory B-cell responses](https://pubmed.ncbi.nlm.nih.gov/3493076/) distinguishes these populations.
Apply it
In a teaching case, a person previously exposed to a particular alloantigen has little detectable circulating antibody before re-exposure, followed by a rapid rise in IgG reactivity afterward. This pattern is consistent with a recall response from immune memory. The initial low serum signal did not directly measure whether antigen-specific memory cells remained.
Common pitfall: A first specimen tested by the laboratory is not necessarily a first immune exposure. A negative serum assay does not establish absence of immune memory.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
50.How do deletion, anergy and regulatory cells contribute to immune tolerance?
Immune tolerance limits responses to particular antigens. Central tolerance includes deletion of strongly self-reactive developing T cells in the thymus. Peripheral tolerance can involve deletion, functional anergy or suppression by regulatory T cells. Anergy leaves a cell present but poorly responsive: antigen recognition without adequate costimulation can induce this state. Regulatory cells restrain responses rather than merely removing circulating antibody. Original studies demonstrate [thymic deletion](https://pubmed.ncbi.nlm.nih.gov/3259471/), [human T-cell anergy without B7 costimulation](https://pubmed.ncbi.nlm.nih.gov/7688125/) and [regulatory-cell maintenance of self-tolerance](https://pubmed.ncbi.nlm.nih.gov/7636184/).
Apply it
T cells in an experimental culture remain viable after recognizing their antigen without B7 costimulation. On a later appropriate antigen challenge, proliferation is markedly reduced. Persistence of viable but unresponsive cells supports anergy; disappearance of the relevant clone would instead support deletion.
Common pitfall: A low antibody result alone does not demonstrate tolerance. Drug-induced suppression and physical antibody removal also differ from established antigen-specific unresponsiveness.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
51.How do inheritance, linkage, linkage disequilibrium and recombination affect HLA patterns?
A child ordinarily inherits one allele at each autosomal HLA locus from each biological parent. Linked loci lie on the same chromosome and tend to be inherited together; meiotic recombination can exchange segments and create new combinations. Linkage disequilibrium describes a population association in which alleles occur together more or less often than expected from their individual frequencies. It is not the same as physical linkage or a recombination event. Family results can support inheritance assessment, but neither a common population haplotype nor an expected parental pattern supplies an unmeasured allele.
Apply it
A parent's two established haplotypes are A1–B8 and A2–B7. A child's supported A1–B7 combination could reflect recombination if phase and inheritance are established. First check identity, typing resolution and family assignments; population linkage disequilibrium alone cannot confirm the child's haplotype or explain away a discrepancy.
Common pitfall: Commonly associated alleles are not inseparable. A child receives contributions from both parents, and incomplete typing can create apparent inheritance conflicts.
NMDP — Matching Blood Stem Cell Donors and Patients · Linkage · ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
52.How does transplant desensitization reduce an antibody barrier without establishing tolerance?
Transplant desensitization seeks to reduce an existing antibody barrier. Plasma exchange removes circulating antibody-containing plasma; it does not directly eliminate all antibody-producing cells. Rituximab targets CD20-bearing B cells, while mature plasma cells generally lack CD20 and can remain. These different targets explain why a lower antibody level after treatment need not persist and why B-cell depletion need not eliminate antibody production. A primary study of [splenic B-cell populations after desensitization](https://pubmed.ncbi.nlm.nih.gov/17241113/) found that plasma cells persisted despite substantial changes in other B-cell populations.
Apply it
A sensitized candidate's antibody reactivity decreases after plasma exchange and rises again later. Continued production by surviving antibody-secreting cells can contribute to this rebound. Compare appropriately timed specimens using comparable assay conditions; the initial fall shows a treatment-associated change in measured reactivity, not permanent immune unresponsiveness.
Common pitfall: Antibody removal, B-cell depletion and tolerance are distinct mechanisms. A post-treatment assay change alone does not establish lasting compatibility or dictate a transplant decision.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
53.How do MHC class I and class II differ in structure, distribution and antigen presentation?
Class I HLA-A, HLA-B and HLA-C molecules contain a polymorphic heavy chain associated with β2-microglobulin; the heavy-chain α1 and α2 domains form the peptide-binding groove. They occur on most nucleated cells. In the conventional endogenous pathway, cytosolic proteins are degraded, peptides pass through TAP into the endoplasmic reticulum and load onto class I for recognition by CD8 T cells. Class II HLA-DR, HLA-DQ and HLA-DP contain transmembrane α and β chains, with the α1 and β1 domains forming the groove. Expression is concentrated on professional antigen-presenting cells, including dendritic cells, B cells and macrophages, and can be induced on other cells. Endocytosed proteins are processed in acidic endosomal compartments. Invariant chain protects the class II groove during trafficking; HLA-DM facilitates replacement of its residual CLIP fragment by peptide for CD4 T-cell recognition. Class I generally binds shorter peptides in a closed-ended groove; class II has an open-ended groove accommodating longer peptides. See original studies of [class I structure](https://pubmed.ncbi.nlm.nih.gov/3309677/) and [HLA-DM-mediated peptide loading](https://pubmed.ncbi.nlm.nih.gov/7606781/).
Apply it
A virus-infected epithelial cell synthesizes viral protein in its cytosol. The conventional pathway is proteolysis, TAP transport and class I loading in the endoplasmic reticulum, followed by CD8 recognition. A dendritic cell processing an engulfed extracellular protein in an acidic vesicle conventionally uses class II and CD4 recognition.
Common pitfall: These are conventional pathways, not absolute source restrictions: cross-presentation can route exogenous antigen to class I. Mature red cells are not examples of typical nucleated class I-bearing targets.
MHC Class II Interaction with CD4 Mediated by a Region Analogous to the MHC Class I Binding Site for CD8 · ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
54.How does calcineurin inhibition suppress immunity without changing HLA genotype?
Tacrolimus binds FKBP, and cyclosporine binds cyclophilin. The resulting drug–protein complexes inhibit calcineurin, a phosphatase involved in lymphocyte activation. Inhibiting this pathway reduces activation-dependent cytokine transcription, including IL-2, and limits T-cell responses. Original studies identified [calcineurin as the shared target](https://pubmed.ncbi.nlm.nih.gov/1715244/) and demonstrated [inhibition of IL-2-related transcriptional activation](https://pubmed.ncbi.nlm.nih.gov/1707162/). This pharmacological suppression does not replace inherited HLA alleles, physically remove all existing antibody or establish donor-specific tolerance.
Apply it
A transplant recipient taking tacrolimus retains the same HLA genotype, and an antibody assay remains reactive. These findings do not contradict calcineurin inhibition: the drug targets activation signaling, while typing measures DNA and the antibody assay measures binding by antibody present in the specimen.
Common pitfall: Immunosuppressed does not mean immunologically unresponsive to every target. A drug's mechanism must be distinguished from the analyte measured by a laboratory assay.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
55.How is HLA-B*57:01 interpreted before abacavir exposure?
HLA-B*57:01 is a pharmacogenetic marker associated with increased risk of abacavir hypersensitivity. The [FDA prescribing information](https://www.accessdata.fda.gov/drugsatfda_docs/pepfar/216223PI.pdf) requires screening before initiation or reinitiation unless a prior documented assessment is available, and contraindicates the abacavir-containing product in HLA-B*57:01-positive patients or those with prior abacavir hypersensitivity. This is an allele-specific risk assessment, not an HLA antibody test. A positive genotype does not prove a reaction has already occurred; a negative genotype does not exclude all hypersensitivity or override a prior reaction.
Apply it
A patient being considered for abacavir has HLA-B*57:01 detected by a valid allele-specific assay. Report the allele and its hypersensitivity-risk significance so the prescriber can select an alternative to the contraindicated product. A separate result reported only as the broader B57 antigen group does not establish the required B*57:01 allele assessment.
Common pitfall: An HLA disease or drug association is not automatically a diagnosis. Here the allele has a specific prescribing consequence, while clinical hypersensitivity remains possible without it.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist · HLA Nomenclature — Naming Alleles
56.How is the stock volume for a target concentration calculated?
For a simple dilution with conserved analyte, use C1V1 = C2V2 after expressing concentrations and volumes in compatible units. V1 is the stock volume required, and V2 is the final total volume, not the diluent volume. Subtract V1 from V2 to obtain the diluent amount. The calculation assumes the stated stock concentration and appropriate mixing; it does not correct an unstable reagent or establish that the resulting preparation is suitable for an assay.
Apply it
To prepare 10 mL at 2 mg/mL from a 20 mg/mL stock, calculate V1 = 1 mL. Combine that stock volume with enough compatible diluent to reach a final total volume of 10 mL.
Common pitfall: Adding 10 mL of diluent to 1 mL of stock produces 11 mL total. Confusing final volume with diluent volume gives the wrong concentration.
Thermo Fisher Scientific — Define and Set Up Standard Dilutions
57.How are cumulative serial dilution factors determined?
The dilution factor for each step equals the final total volume divided by the transferred sample volume. The remaining concentration fraction is its reciprocal: transferred sample volume divided by final total volume. Successive dilution factors multiply, so the cumulative factor is DF1 × DF2 × DF3. Concentration fractions also multiply. Three successive tenfold dilutions therefore give a cumulative dilution factor of 1,000 and a final concentration equal to 1/1,000 of the original. Mix each preparation thoroughly before transferring material to the next step so the transferred aliquot represents that preparation's concentration.
Apply it
Combine 100 µL of specimen with 400 µL of diluent: the final volume is 500 µL, giving a dilution factor of 500/100 = 5. Then transfer 100 µL of this mixed preparation into 300 µL of diluent: the second factor is 400/100 = 4. The cumulative factor is 5 × 4 = 20, while the remaining concentration fraction is (1/5) × (1/4) = 1/20, or 5% of the original concentration.
Common pitfall: Adding the factors gives 9, which is incorrect because the second step dilutes material already diluted in the first step. Keep dilution factor and concentration fraction distinct. Include the transferred sample in the final total volume: 100 µL sample plus 400 µL diluent is a fivefold dilution. State volumes explicitly to avoid confusing sample-to-diluent ratios with sample-to-total-volume ratios.
Thermo Fisher Scientific — Define and Set Up Standard Dilutions · Dilutions: Explanations and Examples of Common Methods
58.How should percentages be calculated from a defined denominator?
A percentage is the specified part divided by the relevant whole, multiplied by 100. Correct interpretation depends on identifying the denominator and ensuring the categories fit the question. In quality monitoring, errors per all reports differ from errors per amended reports. In a simplified two-source chimerism example, donor and recipient contributions can sum to 100% only when those categories exhaust the defined total. State the population or material represented rather than reporting an unexplained percentage.
Apply it
A laboratory finds six transcription errors among 1,200 reports. Calculate 6 divided by 1,200 times 100 = 0.5%, and label the result as the transcription-error percentage among the reports reviewed during that interval.
Common pitfall: Changing the denominator changes the question. A percentage without its population, interval or specimen basis can look precise while remaining clinically or operationally misleading.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist · WHO LQMS Module 14 — Occurrence Management
59.How is molarity calculated from mass and formula mass?
Molarity is moles of solute per liter of final solution. Convert mass to moles by dividing grams by the stated molar mass in grams per mole, then divide by the final volume in liters. Use the correct chemical form and supplied molar mass; related hydrated and anhydrous materials can require different masses. Keep unit cancellation visible so milliliters are not mistaken for liters and mass concentration is not confused with molar concentration.
Apply it
A training calculation uses a solute with molar mass 100 g/mol. Dissolving 0.50 g to a final volume of 0.100 L gives 0.0050 mol divided by 0.100 L, or 0.050 mol/L.
Common pitfall: Dividing grams directly by liters gives mass concentration, not molarity. Using the wrong chemical form's molar mass can also produce a systematically incorrect preparation.
ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist · Thermo Fisher Scientific — Define and Set Up Standard Dilutions
60.How can concentration units be converted without changing meaning?
Convert numerator and denominator units together, using explicit conversion factors. Because one microgram is 1,000 nanograms and one milliliter is 1,000 microliters, 1 ng/µL equals 1 µg/mL. The numerical value stays the same in that particular conversion because both factors change equally. Other conversions do change the number, so do not generalize this coincidence. Check dimensional consistency before comparing concentrations or substituting values into a dilution calculation.
Apply it
A DNA stock is labeled 25 ng/µL, while a worksheet requests µg/mL. Convert it to 25 µg/mL, showing the matching thousandfold numerator and denominator conversions before using the value in the worksheet's dilution formula.
Common pitfall: Converting only the mass unit while leaving the volume unit implicit creates a thousandfold error. Write the complete concentration unit at every calculation stage.
Thermo Fisher Scientific — Define and Set Up Standard Dilutions
What to focus on during revision
The following are CertMedbry study suggestions. Use the official outline to check the full scope of your examination.
- Use the official 18/37/26/36/33 item allocation to prioritize study, giving substantial time to analytical methods, quality systems and general theory.
- Connect each specimen and reagent decision to the analyte being measured, its stability requirements and its traceable identity.
- Distinguish DNA typing, antigen expression, serum antibody specificity, donor-cell binding, ABO grouping and chimerism measurements.
- Practise separating a valid negative result, a technically invalid result and a valid but unresolved result.
- Explain controls mechanistically: what each control establishes, what its failure prevents you from concluding and which results are affected.
- Compare primary and recall responses, tolerance, desensitization and immunosuppression by the cells, molecules and processes each involves.
- Trace class I and class II peptide presentation from protein source through processing and loading to CD8 or CD4 recognition.
- Read HLA names precisely, interpret allele-specific clinical associations and solve concentration, serial dilution, percentage and molarity problems with explicit units.
- Apply concrete CLIA and ASHI obligations within their stated applicability, and distinguish assay verification from establishment of performance specifications.
- Use the full official outline to identify topics beyond these 60 selected concepts; this guide samples the scope rather than replacing it.
A practical preparation plan
1. Map the full outline
Create a study map using the five exact domain names and their official item counts. Mark the concepts covered here, then identify remaining outline topics requiring additional primary educational resources or current authoritative standards.
2. Build a specimen-to-report chain
For each major assay, explain the required material, target, binding or amplification mechanism, controls, interpretable output and report limitations. Include one failure that could occur before testing and one during analysis.
3. Work through discordance
Practise cases involving identity mismatches, failed amplification controls, ABO discrepancies, ambiguous typing and complement interference. State the missing evidence and choose an investigation that can actually resolve the uncertainty.
4. Connect immune mechanisms to cases
Explain a recall antibody response, distinguish anergy from deletion, predict why antibody may persist after B-cell depletion and identify class I versus class II presentation from the peptide source and loading compartment. Interpret an allele-specific pharmacogenetic result without confusing it with antibody detection.
5. Calculate and annotate
Solve dilution and concentration exercises without dropping units. Read example HLA names field by field, retaining expression suffixes and distinguishing protein differences from synonymous or noncoding sequence differences.
6. Review quality decisions
Explain when results must be held, how equipment returns to service, how amendments preserve history and how corrective-action effectiveness is measured. Apply storage-alert and proficiency-testing requirements, then evaluate a hypothetical assay study against its stated acceptance criteria.
7. Use the drills diagnostically
Answer all 20 questions before reading the explanations. For each error, explain why the chosen alternative was tempting, identify the decisive cue and revisit the related concept. Use flashcards to practise the mechanism again after a delay.
Use your sample score as a starting point
The free set contains 20 original practice questions. It is a small learning sample, not an official examination or a validated pass prediction. Review each explanation and its source before moving to another topic.
Start the free sampleOfficial sources and updates
- ACHI Resource Hub
- ACHI Examination Candidate Handbook — January 2026
- ACHI CHA/CHT Exam Content Outline — PDF headed Certified Histocompatibility Technologist
- ACHI CHA, CHS, & CHT Examinations
- WHO LQMS Module 5 — Sample Management
- WHO LQMS Module 4 — Purchasing and Inventory
- WHO LQMS Module 7 — Quantitative Quality Control
- WHO LQMS Module 3 — Equipment
- WHO LQMS Module 16 — Documents and Records
- WHO LQMS Module 14 — Occurrence Management
- WHO LQMS Module 2 — Facilities and Safety
- WHO Laboratory Quality Stepwise Implementation Checklist
- HLA Nomenclature — Naming Alleles
- One Lambda LABScreen Single Antigen HLA Class I — Combi
- One Lambda Micro SSP — Sequence Specific Primer DNA Typing Trays
- Versiti — Crossmatch (Flow Cytometry) Against Recipient
- Canadian Blood Services — Pre-transfusion Testing
- One Lambda Devyser Chimerism for NGS
- HLA Antibody Specification Using Single-antigen Beads—A Technical Solution for the Prozone Effect
- NMDP — Matching Blood Stem Cell Donors and Patients
- MHC Class II Interaction with CD4 Mediated by a Region Analogous to the MHC Class I Binding Site for CD8
- Thermo Fisher Scientific — Define and Set Up Standard Dilutions
- Federal Register :: Request Access
- Federal Register :: Request Access
- 2025_ashi_standards.pdf
- Ultrasensitive chimerism enhances measurable residual disease testing after allogeneic hematopoietic cell transplantation - PMC
- Linkage
- HLA-DM induces CLIP dissociation from MHC class II alpha beta dimers and facilitates peptide loading - PubMed
- Interaction of MHC class I molecules with the transporter associated with antigen processing - PubMed
- Calcineurin is a common target of cyclophilin-cyclosporin A and FKBP-FK506 complexes - PubMed
- 216223PI.pdf
- Dilutions: Explanations and Examples of Common Methods
Confirm current eligibility, fees, scheduling and policies directly with the examination provider. CertMedbry is independent and is not endorsed by these organisations. How we handle content and corrections.