Certified Histocompatibility Technologist (CHT) — Questions and Answers

Question 1: Under CLIA regulations, which category applies to most HLA typing assays performed in histocompatibility labs?

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

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

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

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

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

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

Annual proficiency testing pass rate
Crossmatch result turnaround time from specimen receipt to verbal/electronic report (Correct answer)
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 5: How does EDTA treatment of serum improve HLA antibody detection in Luminex SAB assays?

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

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 6: 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

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

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

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

ABO blood group compatibility only
10/10 allele-level match at HLA-A, -B, -C, -DRB1, and -DQB1 (Correct answer)
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 9: Which feature of HLA class I molecules allows them to present endogenous peptides?

Peptide-binding groove formed by alpha-1 and alpha-2 domains (Correct answer)
Peptide-binding groove formed by alpha-1 and beta-1 domains
Variable CDR3 loop structure
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 10: Which test is used to measure the strength of donor-specific antibodies in mean fluorescence intensity (MFI) units?

Enzyme-linked immunosorbent assay (ELISA)
Mixed lymphocyte reaction (MLR)
Single Antigen Bead (SAB) assay (Correct answer)
Complement-dependent cytotoxicity (CDC) crossmatch

Correct answer: Single Antigen Bead (SAB) assay

SAB assays use Luminex-based beads coated with single HLA antigens to detect and quantify DSA by measuring MFI, providing antibody specificity and relative strength.

Question 11: What is the 'indirect pathway' of allorecognition?

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

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 12: A positive B-cell crossmatch with a concurrent negative T-cell crossmatch most likely indicates:

ABO blood group incompatibility
Anti-HLA class II antibodies directed at DR, DQ, or DP antigens (Correct answer)
Anti-HLA class I antibodies directed at HLA-A, B, or C antigens
Nonspecific complement activation from aggregated immunoglobulin

Correct answer: Anti-HLA class II antibodies directed at DR, DQ, or DP antigens

B cells express both HLA class I and class II; a positive B-cell but negative T-cell crossmatch suggests anti-class II specificity, since T cells lack constitutive HLA class II expression.

Question 13: What is the primary risk of improper specimen labeling in a histocompatibility laboratory?

Minor documentation discrepancy requiring corrective action only
Patient misidentification leading to an incompatible crossmatch result being applied to the wrong patient and fatal transplant error (Correct answer)
Reagent waste from unnecessary repeat testing
Delay in billing processing for the laboratory

Correct answer: Patient misidentification leading to an incompatible crossmatch result being applied to the wrong patient and fatal transplant error

In transplantation, a mislabeled specimen can result in an incorrect crossmatch or HLA type being matched to the wrong patient, potentially causing a fatal incompatible transplant.

Question 14: What is a key aspect of Data Interpretation & Clinical Application?

Ignore calibration
Verbal results
Data Interpretation & Clinical Application standard (Correct answer)
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 15: Which cells are primarily involved in recognizing histocompatibility antigens?

T lymphocytes (Correct answer)
Neurons
Platelets
Red blood cells

Correct answer: T lymphocytes

T lymphocytes, specifically helper T cells and cytotoxic T lymphocytes, are the primary immune cells responsible for recognizing histocompatibility antigens presented by MHC molecules. They play a critical role in mediating cellular immunity and are key players in transplant rejection if the donor and recipient tissues are not compatible.

Question 16: 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 17: Which molecule plays a central role in histocompatibility?

Collagen
ATP
Hemoglobin
HLA (Correct answer)

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 18: Why is HLA typing necessary before transplantation?

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

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 19: What is the primary purpose of running a negative control in a Luminex SAB HLA antibody assay?

To confirm bead lot number accuracy
To verify the positive control exceeded the MFI threshold
To test for complement activation in normal serum
To establish baseline non-specific background fluorescence for result interpretation (Correct answer)

Correct answer: To establish baseline non-specific background fluorescence for result interpretation

Negative controls (normal human serum or buffer) establish the background MFI level; patient MFI values must exceed this background to be considered true positive signals.

Question 20: In a standard CDC crossmatch protocol, which source of complement is most commonly used?

Purified human C3 and C4 components reconstituted from lyophilized stock
Endogenous complement already present in the recipient's test serum
Exogenous rabbit serum added as an external complement source (Correct answer)
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 21: Which organ has the shortest clinically acceptable cold ischemia time, requiring the fastest allocation decisions?

Pancreas
Kidney
Heart (Correct answer)
Liver

Correct answer: Heart

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

Question 22: What type of rejection occurs within minutes of transplant due to preformed antibodies?

Metabolic response
Chronic rejection
Hyperacute rejection (Correct answer)
Cellular adaptation

Correct answer: Hyperacute rejection

Hyperacute rejection is a severe and immediate form of transplant rejection that occurs within minutes to hours after transplantation. It is caused by pre-existing antibodies in the recipient's blood that recognize donor antigens, leading to rapid destruction of the transplanted organ.

Question 23: 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
Proceeding with transplantation under standard triple-drug immunosuppression
Delaying repeat crossmatch testing for six months
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 24: What is a 'laboratory information system' (LIS) and why is it critical in the histocompatibility laboratory?

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
A system exclusively for billing and insurance claims
An instrument control system embedded in analyzers only

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

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

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

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

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

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

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 28: 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 detectable donor antigens on its surface
A kidney with no mismatches at HLA-A, -B, and -DR compared to the recipient (Correct answer)

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

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

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

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

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 31: What is a key aspect of Quality Assurance & Regulatory Compliance?

Verbal results
Ignore calibration
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 32: What is a key aspect of Laboratory Techniques & Testing Procedures?

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

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 33: What is a key aspect of Data Interpretation & Clinical Application?

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

Correct answer: Data Interpretation & Clinical Application standard

Question 34: Chronic allograft nephropathy is most closely associated with which long-term immunological mechanism?

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

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

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

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

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

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 37: What is the primary purpose of performing a pretransplant crossmatch?

To detect preformed donor-specific HLA antibodies in the recipient's serum (Correct answer)
To evaluate donor-recipient body size compatibility
To determine the donor's ABO blood type compatibility
To assess the recipient's renal function before transplant

Correct answer: To detect preformed donor-specific HLA antibodies in the recipient's serum

The pretransplant crossmatch detects preformed donor-specific antibodies (DSAs) in the recipient that could cause hyperacute rejection upon transplantation.

Question 38: In tissue banking, which regulatory body regulates human cells, tissues, and cellular/tissue-based products (HCT/Ps) in the US?

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

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 39: What is the function of the major histocompatibility complex (MHC)?

Transport oxygen
Present antigens to immune cells (Correct answer)
Produce insulin
Generate energy

Correct answer: Present antigens to immune cells

The Major Histocompatibility Complex (MHC) molecules, which include HLAs in humans, are responsible for presenting fragments of proteins (antigens) to T lymphocytes. This presentation is crucial for the immune system to recognize foreign invaders (like pathogens) or abnormal cells, initiating an immune response.

Question 40: What does a prozone effect (hook effect) in Luminex SAB testing refer to?

The assay reaches maximum fluorescence and cannot distinguish concentrations
Complement in serum activates beads producing false positives
High antibody concentrations cause falsely low MFI due to steric inhibition of detection (Correct answer)
Very low antibody concentrations produce falsely elevated MFI

Correct answer: High antibody concentrations cause falsely low MFI due to steric inhibition of detection

In the prozone effect, very high antibody concentrations saturate bead antigen sites and sterically hinder detection antibody binding, leading to falsely reduced MFI values.

Question 41: Pronase treatment of donor lymphocytes before CDC crossmatch testing enhances sensitivity by:

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

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

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

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

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

Accelerated acute rejection
Chronic rejection
Hyperacute rejection (Correct answer)
Acute cellular 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 45: What is the 'direct pathway' of allorecognition?

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

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

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

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 47: A positive T-cell crossmatch using current recipient serum is clinically significant because it indicates:

Recipient sensitization to HLA class II antigens exclusively
The presence of non-HLA autoantibodies in the recipient
Donor-specific anti-HLA class I antibodies in the recipient (Correct answer)
ABO blood group incompatibility between donor and recipient

Correct answer: Donor-specific anti-HLA class I antibodies in the recipient

T cells express HLA class I antigens (A, B, C), so a positive T-cell crossmatch indicates the recipient has preformed antibodies against donor HLA class I antigens.

Question 48: What does the ASHI accreditation standard require regarding competency assessment of laboratory personnel?

Competency assessment only after a quality failure event
Annual documented competency assessment for all personnel performing patient testing (Correct answer)
One-time competency assessment at hire only
Self-assessment by the technologist is sufficient

Correct answer: Annual documented competency assessment for all personnel performing patient testing

ASHI and CLIA require annual documented competency evaluations for all testing personnel covering all procedures they perform, including direct observation and result review.

Question 49: What is the Kidney Donor Profile Index (KDPI) used for in organ allocation?

Measuring kidney function in the recipient post-transplant
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)
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 50: Which immunoglobulin subclass of IgG most efficiently activates complement and is most associated with acute AMR?

IgG4 only
IgG2 only
IgG1 and IgG3 (Correct answer)
IgG2 and IgG4

Correct answer: IgG1 and IgG3

IgG1 and IgG3 are the most potent complement-activating IgG subclasses and are strongly associated with antibody-mediated rejection.

Question 51: What is 'delta check' monitoring in the histocompatibility laboratory?

Comparing a current patient result to a previous result to flag unexpected changes that may indicate an error (Correct answer)
Checking instrument delta calibration before each run
Comparing laboratory results to peer group means
Monitoring room temperature changes that exceed delta thresholds

Correct answer: Comparing a current patient result to a previous result to flag unexpected changes that may indicate an error

Delta checks compare current test results against a patient's prior results; unexpected changes (e.g., new HLA allele appearing) trigger investigation for specimen mix-up or error.

Question 52: What is a key aspect of Data Interpretation & Clinical Application?

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

Correct answer: Data Interpretation & Clinical Application standard

Question 53: In a complement-dependent cytotoxicity (CDC) crossmatch, cell death is assessed by:

Luminex mean fluorescence intensity (MFI) values
Exclusion of a vital dye such as eosin Y or trypan blue (Correct answer)
Flow cytometry using fluorescent cell-surface markers
ELISA optical density readings at 450 nm

Correct answer: Exclusion of a vital dye such as eosin Y or trypan blue

CDC crossmatch uses vital dye exclusion — dead cells with compromised membranes take up the dye, allowing percent cytotoxicity to be calculated.

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

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

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 55: 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 56: What is the purpose of a 'chain of custody' for samples in a histocompatibility laboratory?

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

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

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

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

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)
Performs the transplant surgery in recipient hospitals

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 59: 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 60: Which type of HLA typing method directly determines the nucleotide sequence of HLA alleles?

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

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 61: 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 62: Which assay platform is the current gold standard for HLA antibody detection and specificity determination in US transplant labs?

Western blot
Complement-dependent cytotoxicity (CDC) PRA
ELISA-based PRA panel
Luminex-based Single Antigen Bead (SAB) assay (Correct answer)

Correct answer: Luminex-based Single Antigen Bead (SAB) assay

Luminex SAB assays provide high sensitivity and specificity for detecting and identifying HLA antibodies against individual antigens, making them the standard in most US transplant centers.

Question 63: What does 'cis' configuration refer to in the context of HLA DQ heterodimers?

Homozygous DQ alleles at both loci
Trans-species allele sharing
Alpha and beta chains from opposite parental chromosomes
Alpha and beta chains encoded on the same chromosome (Correct answer)

Correct answer: Alpha and beta chains encoded on the same chromosome

A cis DQ heterodimer is formed by DQA1 and DQB1 chains encoded on the same parental haplotype/chromosome.

Question 64: What term describes the tendency of certain HLA alleles to occur together on the same chromosome more often than expected by chance?

Haplotype shuffling
Linkage disequilibrium (Correct answer)
Codominant expression
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 65: Which HLA class II loci are considered the most polymorphic and clinically significant for transplantation?

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

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

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

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 67: What is a key aspect of Data Interpretation & Clinical Application?

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

Correct answer: Data Interpretation & Clinical Application standard

Question 68: Which feature distinguishes HLA class II molecules from class I in terms of their antigen presentation?

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)
Both present endogenous peptides but to different lymphocyte subsets
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 69: What is a key aspect of Quality Assurance & Regulatory Compliance?

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

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 70: What is a key aspect of Quality Assurance & Regulatory Compliance?

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

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 71: Which complement pathway is most directly activated by donor-specific antibodies (DSA) bound to graft endothelium?

Lectin (MBL) complement pathway
Classical complement pathway (Correct answer)
Terminal pathway only without upstream activation
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 72: 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 73: What is a key aspect of Laboratory Techniques & Testing Procedures?

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

Correct answer: Laboratory Techniques & Testing Procedures standard

Question 74: Which OSHA standard is most directly relevant to worker safety in the histocompatibility laboratory when handling patient specimens?

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

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 75: What is the inheritance pattern of HLA haplotypes?

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

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

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

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

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

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

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

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

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

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 80: 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 81: What is a 'null' allele in HLA nomenclature?

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

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 82: Which chromosome carries the HLA gene complex in humans?

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

Correct answer: Chromosome 6

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

Question 83: What is 'epitope analysis' in the context of HLA antibody characterization?

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

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

CDC
FDA
OSHA
ASHI (Correct answer)

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

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

Correct answer: Quality Assurance & Regulatory Compliance standard

Question 86: What is 'machine perfusion' of donor kidneys designed to accomplish?

Administer immunosuppressants to the organ before transplant
Maintain kidney viability and allow assessment of organ function before transplant by continuously perfusing with solution (Correct answer)
Remove recipient-reactive antibodies from the donor organ
Perform HLA typing on the organ during transport

Correct answer: Maintain kidney viability and allow assessment of organ function before transplant by continuously perfusing with solution

Hypothermic machine perfusion maintains renal viability during preservation, reduces delayed graft function rates, and allows perfusate biomarker assessment of organ quality.

Question 87: In the context of living donor kidney transplantation, what is a 'paired kidney exchange' program?

Incompatible donor-recipient pairs swap donors with other incompatible pairs to create compatible matches (Correct answer)
A program where one donor provides a kidney to two recipients simultaneously
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 88: What does histocompatibility refer to?

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

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 89: Which US law established the framework for organ transplantation, prohibited organ sales, and created the OPTN?

Uniform Determination of Death Act (UDDA) of 1981
National Organ Transplant Act (NOTA) of 1984 (Correct answer)
Americans with Disabilities Act (ADA) of 1990
Health Insurance Portability and Accountability Act (HIPAA) of 1996

Correct answer: National Organ Transplant Act (NOTA) of 1984

NOTA established the legal framework for transplantation in the US, created the OPTN, prohibited the sale of organs, and mandated equitable organ sharing.

Question 90: In NGS-based HLA typing, what is a key advantage over Sanger sequencing?

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

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 91: In a flow cytometric T-cell crossmatch, a positive result is typically defined by:

Greater than 10% of T cells showing cell death by 7-AAD exclusion
Greater than 50% of T cells showing surface antibody binding
A fluorescence channel shift exceeding the laboratory-defined threshold vs. negative control (Correct answer)
An MFI lower than the negative control sample

Correct answer: A fluorescence channel shift exceeding the laboratory-defined threshold vs. negative control

A positive FCXM is defined by a fluorescence shift (in channels or MFI) beyond a laboratory-established threshold relative to the negative control, indicating specific antibody binding.

Question 92: Graft-versus-Host Disease (GVHD) in hematopoietic stem cell transplantation is primarily caused by which donor cells?

Donor macrophages releasing cytokines
Donor T lymphocytes attacking recipient tissues (Correct answer)
Donor NK cells
Donor B cells producing auto-antibodies

Correct answer: Donor T lymphocytes attacking recipient tissues

GVHD occurs when mature donor T cells in the graft recognize host alloantigens as foreign and mount an immune attack against the recipient's tissues.

Question 93: What role do regulatory T cells (Tregs) play in transplant tolerance?

They enhance B cell production of DSA
They directly lyse donor endothelial cells
They suppress effector T cell responses to donor alloantigens, promoting graft acceptance (Correct answer)
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 94: What is the purpose of desensitization protocols in highly sensitized transplant candidates?

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)
To induce tolerance by gradually exposing the patient to donor antigens

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

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

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

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

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

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

Correct answer: Data Interpretation & Clinical Application standard

Question 98: Which molecular typing technique uses allele-specific primers to amplify target HLA sequences?

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

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 99: 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 100: What phenomenon occurs when a heterozygous individual at HLA-DQA1 and HLA-DQB1 can form up to four different DQ heterodimers?

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

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.

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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