Certified Histocompatibility Technologist (CHT) — Questions and Answers

Question 1: Which feature of HLA class I molecules allows them to present endogenous peptides?

Variable CDR3 loop structure
Peptide-binding groove formed by alpha-1 and alpha-2 domains (Correct answer)
Peptide-binding groove formed by alpha-1 and beta-1 domains
Transmembrane domain complexed with beta-2 microglobulin

Correct answer: Peptide-binding groove formed by alpha-1 and alpha-2 domains

HLA class I molecules have a peptide-binding groove formed by the alpha-1 and alpha-2 domains of the heavy chain that accommodates 8–10 amino acid peptides.

Question 2: What is 'epitope analysis' in the context of HLA antibody characterization?

Measuring antibody avidity by competitive ELISA
Identifying the specific molecular surface features (epitopes/eplets) on HLA antigens targeted by antibodies (Correct answer)
Mapping antibodies to HLA gene loci using PCR
Counting the total number of HLA antigens reactive with serum

Correct answer: Identifying the specific molecular surface features (epitopes/eplets) on HLA antigens targeted by antibodies

Epitope analysis uses tools like HLAMatchmaker to identify the specific polymorphic amino acid configurations (eplets) on HLA molecules that are targeted by the patient's antibodies.

Question 3: Why is HLA typing necessary before transplantation?

To monitor vitals
To determine age
To detect blood sugar
To identify compatible tissue (Correct answer)

Correct answer: To identify compatible tissue

HLA typing is essential before transplantation to match the donor and recipient as closely as possible based on their HLA profiles. A good HLA match significantly reduces the likelihood of the recipient's immune system mounting a strong rejection response against the transplanted organ or tissue, thereby improving transplant survival rates.

Question 4: What is the required minimum qualification for a laboratory director of an ASHI-accredited histocompatibility laboratory?

Any doctoral-level scientist with general laboratory management experience
RN with specialized transplant coordinator certification
MT (ASCP) with 5 years of HLA testing experience
PhD or MD with specific training and experience in histocompatibility and immunogenetics (Correct answer)

Correct answer: PhD or MD with specific training and experience in histocompatibility and immunogenetics

ASHI standards require the laboratory director to hold a doctoral degree (PhD or MD) with documented training and competency specifically in histocompatibility and immunogenetics.

Question 5: Which test can confirm the clinical relevance of DSA detected by SAB assay by evaluating actual cell lysis in the presence of complement?

CDC crossmatch (Correct answer)
C1q assay
MLR assay
Flow cytometric crossmatch

Correct answer: CDC crossmatch

The CDC crossmatch uses the patient's serum with donor lymphocytes and complement; cell death confirms the antibody can activate complement and is functionally significant.

Question 6: Under CLIA regulations, which category applies to most HLA typing assays performed in histocompatibility labs?

Waived
Moderate complexity
High complexity (Correct answer)
Provider-performed microscopy

Correct answer: High complexity

HLA typing assays require specialized training, complex interpretation, and quality oversight, placing them in the high complexity category under CLIA 88 regulations.

Question 7: Which OSHA standard is most directly relevant to worker safety in the histocompatibility laboratory when handling patient specimens?

Bloodborne Pathogens Standard (29 CFR 1910.1030) (Correct answer)
Personal Protective Equipment General Requirements
Hazard Communication Standard (HazCom)
Respiratory Protection Standard

Correct answer: Bloodborne Pathogens Standard (29 CFR 1910.1030)

The OSHA Bloodborne Pathogens Standard requires engineering controls, PPE, training, hepatitis B vaccination, and exposure control plans for workers handling blood and tissues.

Question 8: What does histocompatibility refer to?

DNA replication process
Histological staining
Tissue compatibility for transplants (Correct answer)
Blood pressure regulation

Correct answer: Tissue compatibility for transplants

Histocompatibility refers to the degree of similarity between the tissues of a donor and a recipient, which is critical for successful organ or tissue transplantation. A high degree of histocompatibility minimizes the risk of the recipient's immune system recognizing the transplanted tissue as foreign and rejecting it.

Question 9: In tissue banking, which regulatory body regulates human cells, tissues, and cellular/tissue-based products (HCT/Ps) in the US?

UNOS (United Network for Organ Sharing)
FDA (Food and Drug Administration) (Correct answer)
HRSA (Health Resources & Services Administration)
CMS (Centers for Medicare & Medicaid Services)

Correct answer: FDA (Food and Drug Administration)

The FDA regulates human cells, tissues, and cellular/tissue-based products under 21 CFR Part 1271 to prevent transmission of communicable diseases and ensure safety.

Question 10: What term describes the tendency of certain HLA alleles to occur together on the same chromosome more often than expected by chance?

Linkage disequilibrium (Correct answer)
Codominant expression
Haplotype shuffling
Allelic exclusion

Correct answer: Linkage disequilibrium

Linkage disequilibrium describes the non-random association of alleles at two or more loci on the same chromosome within a population.

Question 11: In the context of living donor kidney transplantation, what is a 'paired kidney exchange' program?

A program where one donor provides a kidney to two recipients simultaneously
Incompatible donor-recipient pairs swap donors with other incompatible pairs to create compatible matches (Correct answer)
Living donors are paired with deceased donors for double transplantation
Recipients swap waiting list positions with compatible living donor pairs

Correct answer: Incompatible donor-recipient pairs swap donors with other incompatible pairs to create compatible matches

Kidney paired donation matches incompatible living donor-recipient pairs with other pairs so that each donor gives to a compatible recipient from another pair.

Question 12: In a standard CDC crossmatch protocol, which source of complement is most commonly used?

Endogenous complement already present in the recipient's test serum
Exogenous rabbit serum added as an external complement source (Correct answer)
Purified human C3 and C4 components reconstituted from lyophilized stock
Complement synthesized by the donor lymphocytes during incubation

Correct answer: Exogenous rabbit serum added as an external complement source

Exogenous rabbit serum is the standard complement source in CDC crossmatch because it is potent and consistent; relying on human serum complement yields variable and often insufficient activity.

Question 13: What is the purpose of desensitization protocols in highly sensitized transplant candidates?

To induce tolerance by gradually exposing the patient to donor antigens
To suppress non-HLA antibodies only
To increase cPRA to prioritize organ allocation
To reduce or eliminate DSA levels to enable transplantation across a positive crossmatch (Correct answer)

Correct answer: To reduce or eliminate DSA levels to enable transplantation across a positive crossmatch

Desensitization protocols (e.g., plasmapheresis, IVIG, rituximab) aim to lower DSA titers sufficiently to convert a positive crossmatch to negative, allowing transplantation.

Question 14: Which document describes the step-by-step procedure for performing a test, including safety precautions, reagents, and interpretation criteria?

Standard Operating Procedure (SOP) (Correct answer)
Laboratory Developed Test (LDT) validation report
Corrective Action Report (CAR)
Quality Management Plan (QMP)

Correct answer: Standard Operating Procedure (SOP)

SOPs provide detailed, validated instructions for performing laboratory procedures consistently and are required by CLIA and accreditation standards.

Question 15: Which cell preparation is most appropriate as a target for a T-cell crossmatch?

Washed red blood cells isolated from donor whole blood
Peripheral blood T lymphocytes isolated by rosetting or immunomagnetic separation (Correct answer)
Bone marrow-derived dendritic cells from the donor
Donor-derived platelet-rich plasma

Correct answer: Peripheral blood T lymphocytes isolated by rosetting or immunomagnetic separation

Peripheral blood T lymphocytes are used because they constitutively express HLA class I antigens and are readily isolated from blood by rosetting with neuraminidase-treated sheep RBCs or CD3 magnetic beads.

Question 16: When a Luminex SAB assay shows high MFI on beads from one manufacturer but not another for the same antigen, what is the likely explanation?

The patient is tolerant to that antigen
The assay was performed at different temperatures causing false results
The patient has developed new antibodies
Bead-specific artifacts such as denatured HLA antigen or different antigen density between manufacturers (Correct answer)

Correct answer: Bead-specific artifacts such as denatured HLA antigen or different antigen density between manufacturers

Discrepancies between platforms often reflect bead-specific artifacts including denatured or cryptic antigens on some beads, highlighting the need for confirmatory testing.

Question 17: What is a key aspect of Laboratory Techniques & Testing Procedures?

Verbal results
Ignore calibration
Random sampling
Laboratory Techniques & Testing Procedures standard (Correct answer)

Correct answer: Laboratory Techniques & Testing Procedures standard

A key aspect of Laboratory Techniques & Testing Procedures is adhering to established standards. This ensures accuracy, reliability, and reproducibility of results, which are paramount in a field like histocompatibility where patient lives depend on precise testing.

Question 18: Which immunosuppressive drug class specifically inhibits calcineurin to block IL-2 production and T cell activation?

mTOR inhibitors (sirolimus, everolimus)
Antimetabolites (mycophenolate mofetil)
Corticosteroids
Calcineurin inhibitors (cyclosporine, tacrolimus) (Correct answer)

Correct answer: Calcineurin inhibitors (cyclosporine, tacrolimus)

Cyclosporine and tacrolimus bind cyclophilin and FKBP12 respectively, forming complexes that inhibit calcineurin and block NFAT-mediated IL-2 gene transcription.

Question 19: What does 'cold ischemia time' refer to in organ transplantation?

The total time from donor death to transplant surgery
The time from organ cooling/flushing at procurement until the clamp is removed and blood flow is restored in the recipient (Correct answer)
The time the organ spends at room temperature during transport
The duration of hypothermic machine perfusion only

Correct answer: The time from organ cooling/flushing at procurement until the clamp is removed and blood flow is restored in the recipient

Cold ischemia time begins when the organ is flushed with cold preservation solution and ends at reperfusion in the recipient, representing the total ischemic insult during hypothermic storage.

Question 20: What is a key aspect of Laboratory Techniques & Testing Procedures?

Ignore calibration
Random sampling
Laboratory Techniques & Testing Procedures standard (Correct answer)
Verbal results

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 21: Which technique is commonly used to detect HLA antibodies?

MRI
ELISA (Correct answer)
CT Scan
X-ray

Correct answer: ELISA

Enzyme-Linked Immunosorbent Assay (ELISA) is a widely used laboratory technique to detect and quantify antibodies, including HLA antibodies, in a patient's serum. This method is crucial for pre-transplant screening to assess the recipient's sensitization status and identify potential risks of rejection.

Question 22: Pronase treatment of donor lymphocytes before CDC crossmatch testing enhances sensitivity by:

Inactivating residual IgG antibody present in the serum
Cleaving complement regulatory proteins (CD55, CD59) from the cell surface (Correct answer)
Removing HLA class I antigens to reduce background reactivity
Stripping B-cell surface markers such as CD19 and CD20

Correct answer: Cleaving complement regulatory proteins (CD55, CD59) from the cell surface

Pronase is a protease that removes complement inhibitory proteins (CD55/DAF and CD59/protectin) from the cell surface, making cells more susceptible to complement-mediated lysis.

Question 23: The Amos-modified CDC crossmatch improves specificity over the standard CDC by:

Substituting human complement for rabbit complement
Adding extra rabbit complement to increase lytic activity
Increasing the serum-to-cell ratio in the incubation step
Including wash steps to remove nonspecific serum components before complement addition (Correct answer)

Correct answer: Including wash steps to remove nonspecific serum components before complement addition

Washing steps in the Amos-modified CDC remove unbound antibodies and serum proteins that may activate complement nonspecifically, reducing false-positive results.

Question 24: Which HLA antibody class is most commonly detected in sensitized transplant patients?

IgM
IgA
IgE
IgG (Correct answer)

Correct answer: IgG

IgG HLA antibodies are the most clinically relevant and most commonly detected class in transplant patients, as they cross the placenta, fix complement, and mediate AMR.

Question 25: What is a key aspect of Quality Assurance & Regulatory Compliance?

Verbal results
Random sampling
Quality Assurance & Regulatory Compliance standard (Correct answer)
Ignore calibration

Correct answer: Quality Assurance & Regulatory Compliance standard

Quality Assurance and Regulatory Compliance (QA/RC) fundamentally rely on established standards. These standards define the specific requirements, guidelines, and procedures that must be followed to ensure the quality, safety, and efficacy of products or services. Adhering to these standards is crucial for maintaining compliance with regulatory bodies and achieving consistent, reliable outcomes.

Question 26: Which allocation policy requires that organs be offered locally, then regionally, then nationally?

The emergency status allocation only
The zero-mismatch allocation mandate
The CPRA matching algorithm
The former geographic-based UNOS allocation policy (largely replaced by continuous distribution for some organs) (Correct answer)

Correct answer: The former geographic-based UNOS allocation policy (largely replaced by continuous distribution for some organs)

UNOS historically used local-regional-national geographic sequencing for organ offers, though newer continuous distribution models for hearts and livers prioritize medical urgency and compatibility over geography.

Question 27: What is the inheritance pattern of HLA haplotypes?

Codominant Mendelian inheritance as a haplotype unit (Correct answer)
Mitochondrial inheritance
X-linked dominant
Autosomal recessive only

Correct answer: Codominant Mendelian inheritance as a haplotype unit

HLA haplotypes are inherited as a unit from each parent in a codominant Mendelian fashion, so children express alleles from both parents.

Question 28: The HLA system is the most polymorphic genetic system in the human genome. Which locus currently has the greatest number of recognized alleles?

HLA-B (Correct answer)
HLA-A
HLA-DRB1
HLA-C

Correct answer: HLA-B

HLA-B has the greatest number of recognized alleles of any HLA locus, with thousands catalogued in the IMGT/HLA database.

Question 29: An HLA allele designated HLA-A*02:01 indicates what level of resolution?

Second-field (allele-level) resolution (Correct answer)
First-field (antigen-group) resolution only
Serological split antigen
Protein-level expression status

Correct answer: Second-field (allele-level) resolution

The two-field nomenclature (e.g., 02:01) specifies the allele group (02) and the specific allele within that group (01), representing allele-level resolution.

Question 30: Which organ has the shortest clinically acceptable cold ischemia time, requiring the fastest allocation decisions?

Heart (Correct answer)
Liver
Pancreas
Kidney

Correct answer: Heart

Hearts have a cold ischemia tolerance of only 4–6 hours, requiring rapid allocation and transportation to minimize ischemic injury.

Question 31: What is a 'laboratory information system' (LIS) and why is it critical in the histocompatibility laboratory?

A system exclusively for billing and insurance claims
An instrument control system embedded in analyzers only
A software system that manages patient data, test orders, results, and reports to ensure accurate specimen tracking and result reporting (Correct answer)
A manual log book for recording reagent lot numbers

Correct answer: A software system that manages patient data, test orders, results, and reports to ensure accurate specimen tracking and result reporting

The LIS manages the full workflow from specimen receipt to result release, maintains the patient HLA type history, tracks crossmatch results, and interfaces with transplant center systems.

Question 32: What is the clinical significance of HLA-C typing in solid organ transplantation?

HLA-C mismatches contribute to rejection and are included in some matching algorithms (Correct answer)
HLA-C is only relevant for bone marrow transplantation
HLA-C is not polymorphic and therefore not typed
HLA-C mismatches have no immunological significance

Correct answer: HLA-C mismatches contribute to rejection and are included in some matching algorithms

HLA-C is polymorphic and mismatches at this locus have been associated with rejection outcomes, so many programs include it in compatibility assessments.

Question 33: What is a key aspect of Quality Assurance & Regulatory Compliance?

Quality Assurance & Regulatory Compliance standard (Correct answer)
Random sampling
Ignore calibration
Verbal results

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 34: What role do regulatory T cells (Tregs) play in transplant tolerance?

They suppress effector T cell responses to donor alloantigens, promoting graft acceptance (Correct answer)
They directly lyse donor endothelial cells
They enhance B cell production of DSA
They activate NK cells against the graft

Correct answer: They suppress effector T cell responses to donor alloantigens, promoting graft acceptance

Tregs (CD4+CD25+FoxP3+) suppress alloreactive effector T cells, dampen inflammatory responses, and are central to the induction and maintenance of tolerance.

Question 35: What does it mean when DSA is described as 'complement-binding' in the clinical context?

The antibody blocks complement from injuring the graft
The antibody is detected only by CDC assay
Complement is bound to the antibody preventing it from binding HLA
The antibody can activate the classical complement cascade, increasing rejection risk (Correct answer)

Correct answer: The antibody can activate the classical complement cascade, increasing rejection risk

Complement-binding DSA (e.g., C1q-binding or C3d-fixing) indicates the antibody can activate complement, which is associated with more severe AMR and worse graft outcomes.

Question 36: Which feature distinguishes HLA class II molecules from class I in terms of their antigen presentation?

Both present endogenous peptides but to different lymphocyte subsets
Class II presents endogenous peptides; class I presents exogenous peptides
Class II presents exogenous peptides to CD4+ T cells; class I presents endogenous peptides to CD8+ T cells (Correct answer)
Class II requires β2-microglobulin; class I does not

Correct answer: Class II presents exogenous peptides to CD4+ T cells; class I presents endogenous peptides to CD8+ T cells

HLA class II molecules present peptides derived from extracellular (exogenous) antigens via the endosomal pathway to CD4+ helper T cells.

Question 37: What is the definition of 'brain death' relevant to deceased donor organ procurement?

Irreversible cessation of all functions of the entire brain, including the brainstem (Correct answer)
Irreversible cessation of cardiac function only
Persistent vegetative state without recovery potential
Loss of cortical function with preserved brainstem reflexes

Correct answer: Irreversible cessation of all functions of the entire brain, including the brainstem

Brain death (neurological determination of death) requires irreversible loss of all brain functions including the brainstem, confirmed by clinical examination and often ancillary testing.

Question 38: Which biological safety level (BSL) is appropriate for routine work with human blood and tissue specimens in a histocompatibility lab?

BSL-2 (Correct answer)
BSL-1
BSL-3
BSL-4

Correct answer: BSL-2

BSL-2 practices are standard for clinical labs handling human blood and tissues with potential exposure to bloodborne pathogens like HIV and HBV.

Question 39: A donor-specific antibody (DSA) detected by single antigen bead assay with an MFI of approximately 3,000-5,000 would most likely produce which crossmatch results?

Negative CDC crossmatch but positive flow cytometric crossmatch (Correct answer)
Positive CDC crossmatch and positive flow cytometric crossmatch
Negative CDC crossmatch and negative flow cytometric crossmatch
Positive CDC crossmatch but negative flow cytometric crossmatch

Correct answer: Negative CDC crossmatch but positive flow cytometric crossmatch

Moderate-strength DSAs are detectable by the highly sensitive FCXM but may be below the complement-fixing threshold needed for a positive CDC crossmatch result.

Question 40: What is a key aspect of Data Interpretation & Clinical Application?

Ignore calibration
Data Interpretation & Clinical Application standard (Correct answer)
Verbal results
Random sampling

Correct answer: Data Interpretation & Clinical Application standard

Effective Data Interpretation and Clinical Application are guided by specific standards to ensure accuracy and consistency. These standards provide frameworks and criteria for analyzing results, drawing valid conclusions, and applying them appropriately in patient care or research. Following established standards minimizes errors and optimizes the utility of data in clinical decision-making.

Question 41: What is a key aspect of Quality Assurance & Regulatory Compliance?

Random sampling
Quality Assurance & Regulatory Compliance standard (Correct answer)
Ignore calibration
Verbal results

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 42: Which immune cells are primarily responsible for acute cellular rejection?

NK cells and mast cells
Neutrophils and eosinophils
CD4+ and CD8+ T lymphocytes (Correct answer)
B cells and plasma cells

Correct answer: CD4+ and CD8+ T lymphocytes

Acute cellular rejection is driven predominantly by host CD4+ and CD8+ T cells that recognize donor alloantigens as foreign.

Question 43: In Luminex-based crossmatch (LCM/FCXM by bead), what advantage does it have over traditional CDC crossmatch?

Can detect non-HLA antibodies only
Requires live donor cells for testing
Less expensive and faster turnaround
Higher sensitivity for detecting low-level HLA antibodies (Correct answer)

Correct answer: Higher sensitivity for detecting low-level HLA antibodies

Luminex and flow cytometric crossmatches are significantly more sensitive than CDC, detecting antibody levels too low to activate complement but still potentially harmful.

Question 44: What is a key aspect of Data Interpretation & Clinical Application?

Verbal results
Random sampling
Data Interpretation & Clinical Application standard (Correct answer)
Ignore calibration

Correct answer: Data Interpretation & Clinical Application standard

Question 45: In NGS-based HLA typing, what is a key advantage over Sanger sequencing?

Does not require PCR amplification
Faster turnaround for all clinical cases
Resolves phase ambiguities and provides allele-level phased genotypes (Correct answer)
Requires less DNA template

Correct answer: Resolves phase ambiguities and provides allele-level phased genotypes

NGS typing can resolve cis/trans (phase) ambiguities that Sanger sequencing cannot, producing fully phased allele-level results.

Question 46: Which US federal agency oversees the national organ transplant system and UNOS contract?

Health Resources & Services Administration (HRSA) (Correct answer)
Centers for Medicare & Medicaid Services (CMS)
Centers for Disease Control and Prevention (CDC)
Food and Drug Administration (FDA)

Correct answer: Health Resources & Services Administration (HRSA)

HRSA administers the National Organ Transplant Act (NOTA) and contracts with UNOS to operate the Organ Procurement and Transplantation Network (OPTN).

Question 47: What is a key aspect of Quality Assurance & Regulatory Compliance?

Random sampling
Ignore calibration
Quality Assurance & Regulatory Compliance standard (Correct answer)
Verbal results

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 48: Which type of rejection occurs within minutes to hours after transplantation and is mediated by preformed donor-specific antibodies?

Hyperacute rejection (Correct answer)
Acute cellular rejection
Chronic rejection
Accelerated acute rejection

Correct answer: Hyperacute rejection

Hyperacute rejection is caused by preformed donor-specific antibodies that immediately activate complement and cause thrombosis of graft vasculature.

Question 49: What is a key aspect of Laboratory Techniques & Testing Procedures?

Ignore calibration
Verbal results
Laboratory Techniques & Testing Procedures standard (Correct answer)
Random sampling

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 50: Which of the following best describes 'cross-reactive groups' (CREGs) in HLA antibody analysis?

Groups of HLA antigens sharing common epitopes that may be targeted by the same antibody (Correct answer)
Antigens from different loci that share the same serological name
Groups of patients with similar PRA values
Cross-reactive donor-recipient pairs identified by CDC

Correct answer: Groups of HLA antigens sharing common epitopes that may be targeted by the same antibody

CREGs are sets of HLA antigens that share public epitopes, meaning an antibody to one antigen may cross-react with all antigens in the group due to shared molecular features.

Question 51: In kidney allocation, what does 'CPRA' primarily determine?

Priority points that highly sensitized candidates receive, increasing their chances of receiving an organ (Correct answer)
The organ quality score for deceased donor kidneys
The immunosuppression protocol for post-transplant management
Whether the recipient needs a living versus deceased donor

Correct answer: Priority points that highly sensitized candidates receive, increasing their chances of receiving an organ

High CPRA recipients receive additional priority points in UNOS kidney allocation policy to compensate for their reduced compatible donor pool.

Question 52: In kidney transplantation, which HLA mismatch is considered most detrimental to graft survival?

HLA-DR mismatch (Correct answer)
HLA-DP mismatch only
Blood group O donors to blood group A recipients
HLA-C mismatch only

Correct answer: HLA-DR mismatch

HLA-DR mismatches are consistently associated with the greatest negative impact on kidney graft survival and increased sensitization risk.

Question 53: On the standard NIH CDC scoring scale, a score of 8 indicates:

51–80% cytotoxicity (positive result)
11–20% cytotoxicity (borderline result)
81–100% cytotoxicity (strongly positive result) (Correct answer)
0–10% cytotoxicity (negative result)

Correct answer: 81–100% cytotoxicity (strongly positive result)

The NIH CDC scale runs from 1 (negative, 0–10% dead cells) to 8 (strongly positive, 81–100% dead cells), with higher scores indicating more extensive complement-mediated lysis.

Question 54: What is a key aspect of Quality Assurance & Regulatory Compliance?

Random sampling
Ignore calibration
Verbal results
Quality Assurance & Regulatory Compliance standard (Correct answer)

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 55: Which chromosome carries the HLA gene complex in humans?

Chromosome 7
Chromosome 6 (Correct answer)
Chromosome 12
Chromosome 9

Correct answer: Chromosome 6

The HLA gene complex (MHC) is located on the short arm of chromosome 6 at region 6p21.

Question 56: How does EDTA treatment of serum improve HLA antibody detection in Luminex SAB assays?

It removes lipids that interfere with bead binding
It chelates calcium to inactivate complement, preventing complement-mediated bead interference (Correct answer)
It reduces background fluorescence from non-specific binding
It denatures IgM antibodies to isolate IgG only

Correct answer: It chelates calcium to inactivate complement, preventing complement-mediated bead interference

EDTA chelates calcium ions required for complement activation, preventing complement already present in serum from binding C1q to beads and generating false-positive signals.

Question 57: Which molecule plays a central role in histocompatibility?

HLA (Correct answer)
ATP
Collagen
Hemoglobin

Correct answer: HLA

Human Leukocyte Antigens (HLAs) are proteins found on the surface of most cells in the body, playing a vital role in the immune system's ability to distinguish self from non-self. These highly polymorphic molecules are the primary determinants of tissue compatibility, making them central to histocompatibility and transplant success.

Question 58: What is a key aspect of Laboratory Techniques & Testing Procedures?

Verbal results
Random sampling
Laboratory Techniques & Testing Procedures standard (Correct answer)
Ignore calibration

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 59: What is a 'zero-antigen mismatch' kidney in UNOS allocation policy?

A kidney from an HLA-identical sibling donor
A kidney matched only at blood group with no HLA typing done
A kidney with no mismatches at HLA-A, -B, and -DR compared to the recipient (Correct answer)
A kidney with no detectable donor antigens on its surface

Correct answer: A kidney with no mismatches at HLA-A, -B, and -DR compared to the recipient

Zero-antigen mismatch (0-MM) kidneys have no mismatches at HLA-A, -B, and -DR antigens between donor and recipient and are shared nationally under UNOS policy due to superior outcomes.

Question 60: What is the primary function of beta-2 microglobulin (β2m) in HLA class I molecules?

Signals intracellularly after antigen recognition
Stabilizes the alpha chain and is required for surface expression (Correct answer)
Directly binds the peptide antigen
Anchors the molecule to the cell membrane

Correct answer: Stabilizes the alpha chain and is required for surface expression

β2m is a non-covalently associated light chain that stabilizes the HLA class I alpha chain and is essential for proper folding and surface expression.

Question 61: What is a key aspect of Quality Assurance & Regulatory Compliance?

Ignore calibration
Verbal results
Quality Assurance & Regulatory Compliance standard (Correct answer)
Random sampling

Correct answer: Quality Assurance & Regulatory Compliance standard

A key aspect of Quality Assurance & Regulatory Compliance is adhering to established standards. This ensures that all laboratory processes meet regulatory requirements, maintain high quality, and produce reliable results, which is critical for patient safety and accreditation in histocompatibility testing.

Question 62: Which best describes an auto-crossmatch and its clinical use?

Testing archived historical serum against current donor cells
Testing donor serum against recipient lymphocytes in reverse
Testing recipient serum against cells from a random unrelated donor
Testing recipient serum against the recipient's own lymphocytes (Correct answer)

Correct answer: Testing recipient serum against the recipient's own lymphocytes

An auto-crossmatch tests recipient serum against the recipient's own cells to detect autoantibodies, which can cause false-positive crossmatch results if not identified.

Question 63: What is a key aspect of Laboratory Techniques & Testing Procedures?

Verbal results
Random sampling
Laboratory Techniques & Testing Procedures standard (Correct answer)
Ignore calibration

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 64: Chronic allograft nephropathy is most closely associated with which long-term immunological mechanism?

Persistent donor-specific antibody (DSA)-mediated injury (Correct answer)
NK cell cytotoxicity only
Autoimmune anti-tubular basement membrane antibodies
Recurrent hyperacute rejection episodes

Correct answer: Persistent donor-specific antibody (DSA)-mediated injury

Chronic antibody-mediated rejection driven by DSA leads to progressive microvascular injury, transplant glomerulopathy, and eventual graft loss.

Question 65: What is a 'virtual crossmatch' in transplantation?

A crossmatch performed using virtual reality imaging equipment
A crossmatch using stored serum from a previous transplant
A computer-simulated rejection model
Predicting crossmatch outcome by comparing recipient DSA specificities to donor HLA typing without a physical test (Correct answer)

Correct answer: Predicting crossmatch outcome by comparing recipient DSA specificities to donor HLA typing without a physical test

Virtual crossmatch uses recipient antibody specificity data (SAB testing) and donor HLA typing to predict whether a physical crossmatch would be positive, enabling faster organ allocation.

Question 66: What is the significance of turnaround time (TAT) monitoring in a deceased donor histocompatibility laboratory?

Rapid TAT is critical because organs have limited viability and allocation decisions cannot wait (Correct answer)
TAT monitoring is only used for billing purposes
TAT is only important for elective living donor workups
TAT standards are set by individual labs with no external requirements

Correct answer: Rapid TAT is critical because organs have limited viability and allocation decisions cannot wait

Deceased donor typing, crossmatching, and antibody results must be reported rapidly because organ ischemia time is limited and allocation is time-critical.

Question 67: Which organization accredits histocompatibility laboratories in the U.S.?

FDA
ASHI (Correct answer)
OSHA
CDC

Correct answer: ASHI

The American Society for Histocompatibility and Immunogenetics (ASHI) is the primary organization that provides accreditation for histocompatibility and immunogenetics laboratories in the United States. ASHI accreditation ensures that laboratories meet stringent quality standards and proficiency requirements for testing critical to organ transplantation.

Question 68: Which accreditation organization specifically accredits histocompatibility laboratories in the United States?

American Society for Histocompatibility and Immunogenetics (ASHI) (Correct answer)
Clinical Laboratory Improvement Amendments (CLIA) only
Joint Commission (TJC)
College of American Pathologists (CAP) only

Correct answer: American Society for Histocompatibility and Immunogenetics (ASHI)

ASHI provides specialty accreditation for histocompatibility laboratories, setting standards specific to HLA testing, crossmatching, and transplant immunology beyond CLIA requirements.

Question 69: What is a key aspect of Quality Assurance & Regulatory Compliance?

Quality Assurance & Regulatory Compliance standard (Correct answer)
Ignore calibration
Verbal results
Random sampling

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 70: Which molecular typing technique uses allele-specific primers to amplify target HLA sequences?

SSOP (Sequence-Specific Oligonucleotide Probes)
SBT (Sequence-Based Typing)
SSP (Sequence-Specific Primers) (Correct answer)
Flow cytometry crossmatch

Correct answer: SSP (Sequence-Specific Primers)

SSP uses primers designed to amplify only specific alleles or allele groups under defined PCR conditions, providing rapid low-to-intermediate resolution typing.

Question 71: What action should be taken when a reagent lot number changes for a critical assay in a histocompatibility laboratory?

Report the lot change to the accrediting body before use
Immediately discard the old lot and begin using the new lot
Perform parallel testing with the old and new lot to verify equivalent performance before clinical use (Correct answer)
Request a waiver from CLIA before implementing the new lot

Correct answer: Perform parallel testing with the old and new lot to verify equivalent performance before clinical use

Lot-to-lot verification ensures that the new reagent lot performs equivalently to the current lot on known samples before being used for clinical testing.

Question 72: What is a key aspect of Laboratory Techniques & Testing Procedures?

Verbal results
Ignore calibration
Random sampling
Laboratory Techniques & Testing Procedures standard (Correct answer)

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 73: What is a key aspect of Quality Assurance & Regulatory Compliance?

Quality Assurance & Regulatory Compliance standard (Correct answer)
Ignore calibration
Verbal results
Random sampling

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 74: Which type of HLA typing method directly determines the nucleotide sequence of HLA alleles?

Low-resolution SSP
Complement-dependent cytotoxicity
Sequence-Based Typing (SBT/SBT-NGS) (Correct answer)
Serological microlymphocytotoxicity

Correct answer: Sequence-Based Typing (SBT/SBT-NGS)

SBT (Sanger sequencing) and NGS-based methods directly read the nucleotide sequence to assign high-resolution HLA alleles.

Question 75: Which quality indicator is most important to monitor specifically for urgent deceased donor crossmatch testing?

Crossmatch result turnaround time from specimen receipt to verbal/electronic report (Correct answer)
Annual proficiency testing pass rate
Monthly technologist training hours
Number of reagent lots used per quarter

Correct answer: Crossmatch result turnaround time from specimen receipt to verbal/electronic report

For deceased donor crossmatches, TAT from specimen receipt to final result is the key quality indicator because transplant surgeons cannot proceed until results are available and organs cannot wait.

Question 76: What does a positive C4d staining on peritubular capillaries in a kidney biopsy indicate in the transplant setting?

Acute cellular rejection only
Viral nephropathy
Antibody-mediated rejection (AMR) (Correct answer)
Calcineurin inhibitor toxicity

Correct answer: Antibody-mediated rejection (AMR)

Peritubular capillary C4d deposition is a marker of complement activation by DSA and is a diagnostic criterion for antibody-mediated rejection.

Question 77: What is a key aspect of Data Interpretation & Clinical Application?

Data Interpretation & Clinical Application standard (Correct answer)
Random sampling
Ignore calibration
Verbal results

Correct answer: Data Interpretation & Clinical Application standard

Question 78: Which allocation principle ensures that the sickest patients (greatest medical urgency) receive priority for certain organs like hearts and livers?

Medical urgency status (e.g., Status 1A/1B for hearts, MELD score for livers) (Correct answer)
Zero-mismatch HLA compatibility preference
Waiting time accrued since listing
Geographic proximity to the donor hospital

Correct answer: Medical urgency status (e.g., Status 1A/1B for hearts, MELD score for livers)

Hearts and livers use medical urgency scoring (Status 1A/1B and MELD/PELD respectively) so that the most critically ill patients on life support or near death receive priority.

Question 79: What is a key aspect of Data Interpretation & Clinical Application?

Data Interpretation & Clinical Application standard (Correct answer)
Ignore calibration
Random sampling
Verbal results

Correct answer: Data Interpretation & Clinical Application standard

Question 80: A 'virtual crossmatch' in transplantation is performed by:

Generating in vitro donor cell lines from stored DNA for physical testing
Comparing recipient antibody specificities (from solid-phase assay) against the donor's HLA type (Correct answer)
Testing recipient serum against a virtual panel of 100 random donor cells
Using computer modeling to simulate complement-mediated lysis in silico

Correct answer: Comparing recipient antibody specificities (from solid-phase assay) against the donor's HLA type

Virtual crossmatch matches the recipient's HLA antibody profile (from Luminex single antigen bead testing) against the donor's HLA typing to predict physical crossmatch outcome without requiring donor cells.

Question 81: Which HLA class II loci are considered the most polymorphic and clinically significant for transplantation?

HLA-DPA1 and HLA-DPB1
HLA-DOA and HLA-DOB
HLA-DRB1 and HLA-DQB1 (Correct answer)
HLA-DMA and HLA-DMB

Correct answer: HLA-DRB1 and HLA-DQB1

HLA-DRB1 and HLA-DQB1 are the most polymorphic class II loci and are routinely typed for solid organ transplantation.

Question 82: What is the purpose of an Internal Proficiency Testing (IPT) program when no approved external PT program is available for a test?

The lab may skip proficiency testing for non-covered analytes
The lab must obtain a CLIA waiver to continue performing the test
The lab uses split samples, previously tested specimens, or reference materials to verify ongoing accuracy (Correct answer)
The lab director self-certifies competency in lieu of formal PT

Correct answer: The lab uses split samples, previously tested specimens, or reference materials to verify ongoing accuracy

When external PT is unavailable, CLIA requires labs to use an equivalent process such as split-sample testing or reference material analysis twice per year.

Question 83: What is a key aspect of Data Interpretation & Clinical Application?

Verbal results
Ignore calibration
Random sampling
Data Interpretation & Clinical Application standard (Correct answer)

Correct answer: Data Interpretation & Clinical Application standard

Question 84: Which complement pathway is most directly activated by donor-specific antibodies (DSA) bound to graft endothelium?

Classical complement pathway (Correct answer)
Terminal pathway only without upstream activation
Lectin (MBL) complement pathway
Alternative complement pathway

Correct answer: Classical complement pathway

DSA bound to donor endothelial HLA antigens activate the classical complement pathway via C1q, leading to C4d deposition.

Question 85: What is the purpose of a 'chain of custody' for samples in a histocompatibility laboratory?

To authorize test ordering by clinicians
To document the handling, transfer, and testing of specimens to ensure traceability and integrity (Correct answer)
To record refrigerator temperature logs for specimen storage
To track reagent lot numbers used for each test

Correct answer: To document the handling, transfer, and testing of specimens to ensure traceability and integrity

Chain of custody documentation tracks every person who handled a specimen and every step from collection to result, ensuring traceability and defending result integrity.

Question 86: Which HLA gene is encoded outside the classical MHC class I region yet is important in NK cell regulation?

HLA-C
HLA-E (Correct answer)
HLA-A
HLA-B

Correct answer: HLA-E

HLA-E is a non-classical class I molecule encoded within the MHC that presents leader-sequence peptides and serves as a primary ligand for NK cell NKG2 receptors.

Question 87: What is the significance of 'de novo' donor-specific antibodies (dnDSA) post-transplant?

They are strongly associated with late antibody-mediated rejection and graft loss (Correct answer)
They indicate successful tolerance induction
They are a normal immune response with no clinical consequence
They only form against blood group antigens

Correct answer: They are strongly associated with late antibody-mediated rejection and graft loss

De novo DSA developing after transplantation are a major risk factor for chronic antibody-mediated rejection and long-term graft failure.

Question 88: Which crossmatch technique has the LEAST sensitivity for detecting donor-specific antibodies?

Standard CDC crossmatch without wash steps (Correct answer)
AHG-enhanced CDC crossmatch
Flow cytometric crossmatch (FCXM)
Luminex single antigen bead (SAB) assay

Correct answer: Standard CDC crossmatch without wash steps

Standard CDC crossmatch requires sufficient antibody to activate the full complement cascade; it is the least sensitive, while SAB, FCXM, and AHG-CDC can all detect lower antibody concentrations.

Question 89: In the context of a living donor kidney transplant, a positive T-cell CDC crossmatch using current recipient serum most commonly leads to:

Proceeding with transplantation but using only B-cell crossmatch results for decision-making
Delaying repeat crossmatch testing for six months
Proceeding with transplantation under standard triple-drug immunosuppression
Avoidance of transplantation or initiation of a desensitization protocol (Correct answer)

Correct answer: Avoidance of transplantation or initiation of a desensitization protocol

A positive T-cell CDC crossmatch signifies high risk of hyperacute or acute humoral rejection; transplantation is typically contraindicated unless the patient undergoes successful desensitization.

Question 90: Which HLA matching criterion is most critical for unrelated hematopoietic stem cell transplantation (HCT) to minimize GVHD?

10/10 allele-level match at HLA-A, -B, -C, -DRB1, and -DQB1 (Correct answer)
ABO blood group compatibility only
4-antigen match for HLA-A and -B only
HLA-DPB1 matching only

Correct answer: 10/10 allele-level match at HLA-A, -B, -C, -DRB1, and -DQB1

For unrelated HCT, 10/10 allele-level matching at A, B, C, DRB1, and DQB1 is the standard of care to minimize GVHD risk and improve survival.

Question 91: What is the role of an Organ Procurement Organization (OPO) in the transplant system?

Performs the transplant surgery in recipient hospitals
Provides immunosuppressive drugs to transplant recipients
Manages the national transplant waiting list
Coordinates organ donation, donor evaluation, and procurement from deceased donors in a defined service area (Correct answer)

Correct answer: Coordinates organ donation, donor evaluation, and procurement from deceased donors in a defined service area

OPOs are federally designated organizations responsible for organ donor identification, family consent, medical management of brain-dead donors, and organ procurement in their DSA.

Question 92: What is the primary purpose of HLA typing in the context of deceased donor organ allocation?

To diagnose the cause of brain death in the donor
To identify compatible recipients, perform virtual crossmatches, and prioritize zero-mismatch candidates (Correct answer)
To assess donor organ quality before procurement
To determine ABO blood type for compatibility

Correct answer: To identify compatible recipients, perform virtual crossmatches, and prioritize zero-mismatch candidates

Rapid deceased donor HLA typing allows the allocation center to identify compatible candidates, perform virtual crossmatches, flag DSA concerns, and apply zero-mismatch priority rules.

Question 93: What is a key aspect of Quality Assurance & Regulatory Compliance?

Random sampling
Quality Assurance & Regulatory Compliance standard (Correct answer)
Ignore calibration
Verbal results

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 94: What is the 'direct pathway' of allorecognition?

B cells directly bind unprocessed donor HLA
Host T cells recognize intact donor HLA-peptide complexes displayed on donor APCs (Correct answer)
NK cells lyse target cells lacking self-HLA
Host T cells process donor HLA peptides via self APCs

Correct answer: Host T cells recognize intact donor HLA-peptide complexes displayed on donor APCs

In the direct pathway, recipient T cells recognize intact allogeneic HLA molecules (with bound peptide) directly on donor-derived APCs, driving acute rejection.

Question 95: What is the Kidney Donor Profile Index (KDPI) used for in organ allocation?

Calculating recipient priority on the transplant waiting list
Estimating the relative risk of graft failure from a deceased donor kidney compared to the average donor (Correct answer)
Measuring kidney function in the recipient post-transplant
Determining living donor eligibility for donation

Correct answer: Estimating the relative risk of graft failure from a deceased donor kidney compared to the average donor

KDPI uses 10 donor characteristics to estimate a percentile ranking of graft failure risk, helping recipients and transplant teams make informed decisions about marginal-quality organs.

Question 96: In the context of sensitization, what does a high Calculated Panel Reactive Antibody (cPRA) value indicate?

The patient has a strong T cell response to alloantigens
The patient has antibodies reactive to a large proportion of the donor pool, making compatible donors rare (Correct answer)
The patient has low risk of rejection after transplant
The patient is tolerant to most HLA mismatches

Correct answer: The patient has antibodies reactive to a large proportion of the donor pool, making compatible donors rare

A high cPRA (e.g., 90%+) means the recipient has HLA antibodies that would react with most potential donors, significantly limiting compatible organ availability.

Question 97: What is the 'indirect pathway' of allorecognition?

Host T cells directly recognize intact donor HLA molecules on donor cells
B cells produce antibodies against donor blood group antigens
NK cells recognize missing self HLA on donor cells
Host T cells recognize donor peptides presented by self HLA on host APCs (Correct answer)

Correct answer: Host T cells recognize donor peptides presented by self HLA on host APCs

In the indirect pathway, donor HLA proteins are processed into peptides and presented by recipient APCs on self-HLA molecules to recipient T cells.

Question 98: What phenomenon occurs when a heterozygous individual at HLA-DQA1 and HLA-DQB1 can form up to four different DQ heterodimers?

MHC restriction
Haplotype imbalance
Trans (in-trans) DQ pairing (Correct answer)
Allelic exclusion

Correct answer: Trans (in-trans) DQ pairing

In heterozygous individuals, DQA1 and DQB1 chains from opposite chromosomes can pair in trans, generating up to four different DQ heterodimers.

Question 99: What is a 'null' allele in HLA nomenclature?

An allele encoding only intracellular protein
An allele with a stop codon or splice mutation resulting in no cell-surface expression (Correct answer)
An allele shared by all human populations
An allele with no sequence polymorphism vs. reference

Correct answer: An allele with a stop codon or splice mutation resulting in no cell-surface expression

Null alleles (designated 'N' in the allele name) harbor mutations such as premature stop codons that prevent surface expression of the HLA protein.

Question 100: Which type of HLA antibody specificity is considered most critical to identify before transplantation?

Anti-HLA-DP antibodies only
MICA antibodies only
Donor-specific antibodies (DSA) directed against the specific donor's HLA antigens (Correct answer)
Any HLA class I antibody regardless of donor type

Correct answer: Donor-specific antibodies (DSA) directed against the specific donor's HLA antigens

DSA against the specific donor's HLA antigens are the most clinically critical to identify, as they predict positive crossmatches and risk for hyperacute and AMR.

Certified Histocompatibility Technologist (CHT)

The CHT certification validates the knowledge and skills of professionals performing histocompatibility testing, crucial for organ transplantation and disease association studies.

100

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150 min

Time Limit

70%

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