Compatibility testing, also known as crossmatching, is a critical step before any blood transfusion. It ensures that the donor’s red blood cells are serologically compatible with the recipient’s plasma to prevent transfusion reactions.
This chapter explains the principles, techniques, and interpretation of compatibility testing — including immediate-spin, antiglobulin, and electronic crossmatch methods — all of which ensure the safety and efficacy of blood transfusions.
Topics Covered in This Chapter
- Purpose and principles of compatibility testing
- Pretransfusion testing steps (ABO/Rh typing, antibody screen, crossmatch)
- Types of crossmatch:
- Immediate-spin crossmatch
- Indirect antiglobulin (AHG) crossmatch
- Computer (electronic) crossmatch
- Compatibility testing for special cases: neonates, transfusion reactions, and emergencies
- Handling incompatible results and resolving discrepancies
- Sample requirements, labeling, and validity period
- Emergency release procedures and documentation
Why It Matters
Compatibility testing is the final safeguard in transfusion medicine. A correctly performed crossmatch ensures that the donor’s blood is safe and compatible for the recipient, preventing acute hemolytic transfusion reactions, which can be fatal.
Knowledge of crossmatching principles is vital for Medical Laboratory Scientists who perform pretransfusion testing in hospitals and blood banks.
Learning Outcomes
After studying this section, learners should be able to:
- Explain the purpose and principles of compatibility testing.
- Describe the different types of crossmatch techniques.
- Interpret compatibility test results accurately.
- Manage and document emergency blood release procedures.
- Recognize and troubleshoot incompatible results.
60 MCQs (4601 – 4660):
- What is the primary purpose of the major crossmatch?
a) To detect unexpected antibodies in the donor unit
b) To confirm the ABO group of the donor
c) To detect recipient antibodies directed against antigens on donor red cells
d) To identify irregular antibodies in the recipient’s serum - A patient is group A, Rh-positive but has a positive direct antiglobulin test (DAT). Which of the following situations would occur during compatibility testing?
a) All major crossmatches would be incompatible
b) The weak D test and control would be positive
c) The antibody screening test would be positive
d) The forward and reverse ABO groupings would not agree - Which of the following would most likely be responsible for an incompatible antiglobulin crossmatch?
a) Recipient’s red cells possess a low-frequency antigen
b) Anti-K antibody in donor serum
c) Recipient’s red cells are polyagglutinable
d) Donor red cells have a positive direct antiglobulin test - A computer (electronic) crossmatch can only be used when which of the following conditions is met?
a) The patient has a historical ABO and Rh in the computer system
b) The computer system manual states such application is acceptable
c) The patient fails to demonstrate clinically significant alloantibodies
d) The immediate spin (IS) crossmatch is also negative - A unit of Red Blood Cells, Leukocytes Reduced is released and returned to the transfusion service within the hour. The temperature is 12°C, the unit is unentered, and segments are attached. What action should be taken?
a) Discard the unit as it was released
b) Discard the unit as temperature was not maintained
c) Accept the unit into inventory since it was not entered
d) Accept the unit since it is visually acceptable with segments - When completing a pretransfusion sample, the recipient’s ABO testing must include which reagents?
a) Anti-A,B; A1 cells; B cells; O cells
b) Anti-A; anti-B; A1 cells; A2 cells; B cells
c) Anti-A; anti-B; A1 cells; B cells
d) Anti-A; anti-B; anti-A,B; A1 cells; A2 cells; B cells - What is the purpose of the immediate spin (IS) phase of the crossmatch?
a) Detect clinically significant alloantibodies in recipient
b) Detect ABO incompatibility between donor and recipient
c) Verify the correct blood sample from recipient was collected
d) Verify the presence of IgM alloantibodies in the recipient - A pretransfusion sample from a patient with an unknown transfusion history is collected on Tuesday at 4:00 PM. How long is this sample valid for crossmatching?
a) Wednesday at 4:00 PM
b) Thursday at 11:59 PM
c) Friday at 11:59 PM
d) Saturday at 4:00 PM - In an emergency release situation, what must be on the label or tie-tag of the issued unit?
a) The name of the individual releasing the units
b) A statement that compatibility testing has not been completed
c) A statement that the patient’s ABO and Rh type are unknown
d) Physician’s statement and signature requesting emergency release - For a patient who is group A 22B with an anti-A1, which blood type is the first choice if A 22B units are unavailable?
a) B, Rh-positive
b) B, Rh-negative
c) A
11B, Rh-positive
d) O, Rh-negative - A 29-year-old male is hemorrhaging severely. He is AB, Rh-negative. Six units are required STAT. Which available type is most preferable?
a) AB, Rh-positive
b) A, Rh-negative
c) A, Rh-positive
d) O, Rh-negative - A patient received 4 units of blood 2 years previously and now has multiple antibodies. What would be helpful in determining the specificities of these antibodies?
a) Phenotype his cells to determine which alloantibodies he can produce
b) Recommend the use of directed donors
c) Use proteolytic enzymes to treat the patient’s red cells
d) Use the patient’s serum for antigen typing of compatible units - A 10% red cell suspension in saline is used in a compatibility test. What is the most likely result?
a) False-positive due to antigen excess
b) False-positive due to the prozone phenomenon
c) False-negative due to the prozone phenomenon
d) False-negative due to antigen excess - Which of the following must be verified immediately before starting a transfusion?
a) Name of the donation facility
b) Date and time the crossmatch was completed
c) Patient CBC results and date of last transfusion
d) Donation identification number and donor’s ABO and Rh - A patient has an order for a type and screen and 6 units of Red Blood Cells. At the IAT phase, both antibody detection cells and 2 of 6 crossmatched units are incompatible. What is the most likely cause?
a) Recipient alloantibody
b) Recipient autoantibody
c) Donors have positive DATs
d) Recipient has a positive DAT - A patient is readmitted 3 weeks post-transfusion with a hemoglobin of 7 g/dL. Initial tests: ABO/Rh A+; antibody screen negative; DAT 1+ mixed field. What is the next step?
a) Antibody identification panel on the patient’s serum
b) Repeat the ABO type on the donor units
c) Perform an elution and identify the antibody in the eluate
d) Crossmatch the post-reaction serum with the donor units - For neonates less than 4 months old, preliminary blood bank testing must include:
a) ABO forward typing and ABO reverse typing
b) ABO reverse typing and testing for K antigen
c) ABO reverse typing and antibody detection testing
d) ABO forward typing and Rh testing for D antigen - A transfusion request must show all of the following EXCEPT:
a) ABO and Rh type of component requested
b) Name of ordering physician
c) Date blood typing was performed
d) Two recipient identifiers - In a massive transfusion protocol, what other components are routinely administered with Red Blood Cells?
a) Cryoprecipitated AHF and Apheresis Platelets, Irradiated
b) Fresh Frozen Plasma and Apheresis Platelets
c) Deglycerolized Red Blood Cells and Fresh Frozen Plasma
d) Granulocytes and Cryoprecipitated AHF - A patient’s record shows a previous anti-Jk bb, but the current antibody screen is negative. What should be done before transfusion?
a) Phenotype the patient’s red cells for the Jk
bb antigen
b) Perform a cell panel on the patient’s serum
c) Crossmatch ABO type specific units and release compatible units
d) Phenotype donor units and select Jk(b–) units for compatibility testing - A patient is group B, Rh-negative and requires 2 units of Fresh Frozen Plasma. Which unit would be most acceptable?
a) Group O, Rh-negative
b) Group A, Rh-negative
c) Group B, Rh-positive
d) Group AB, Rh-positive - Which of the following is proper procedure for preparing Platelets from Whole Blood?
a) Light spin followed by a hard spin
b) Light spin followed by 2 hard spins
c) 2 light spins
d) Hard spin followed by a light spin - A unit of Red Blood Cells that expires in 32 days is irradiated. What is the new expiration date?
a) Remain the same
b) Reduced by 4 days
c) Reduced by 14 days
d) Increased by 2 days - What is the maximum allowable storage period for a unit of Red Blood Cells if the seal is broken and it is stored at 1-6°C?
a) 6 hours
b) 24 hours
c) 48 hours
d) 72 hours - A patient with a platelet count of 10,000/µL receives one unit of Apheresis Platelets, Leukocytes Reduced. The post-transfusion platelet count is 50,000/µL. These results indicate:
a) The presence of HLA antibodies and refractoriness
b) The patient is actively bleeding
c) The platelet count is the expected posttransfusion increment
d) The patient’s pre-transfusion platelet count was incorrect - For a 70-kg patient, what is the expected increase in platelet count from one unit of Apheresis Platelets?
a) 5,000-10,000/µL
b) 10,000-20,000/µL
c) 20,000-25,000/µL
d) 30,000-50,000/µL - Which of the following is an indication for using Washed Red Blood Cells?
a) A pregnant woman with a history of HDN
b) An IgA-deficient patient with a history of anaphylaxis
c) A patient with a positive DAT and red cell autoantibody
d) A newborn with a hematocrit of <30% - A patient with a hemoglobin of 7.0 g/dL receives 2 units of Red Blood Cells. What is the expected posttransfusion hemoglobin?
a) 8.0 g/dL
b) 9.0 g/dL
c) 10.0 g/dL
d) 11.0 g/dL - Which of the following practices helps minimize bacterial contamination of platelet products?
a) Use of an 18-gauge needle
b) Diversion pouch
c) Green soap scrub
d) UV irradiation - Prior to initiating a blood transfusion, the transfusionist and another qualified individual must:
a) Match the blood component to the recipient using 2 independent identifiers
b) Verify that the recipient’s vital signs are within normal limits
c) Review the patient’s medical record to verify the need for a transfusion
d) Order a baseline hemoglobin and hematocrit before the transfusion is started - 1. The primary purpose of compatibility testing is to:
a) Confirm ABO and Rh grouping
b) Detect antibodies that could cause transfusion reactions
c) Identify donor hemoglobin concentration
d) Ensure platelet function in donor blood - Which of the following tests is performed first in pretransfusion testing?
a) Crossmatch
b) Antibody screen
c) ABO and Rh typing
d) DAT - The term “crossmatch” refers to:
a) Testing donor serum against recipient red cells
b) Testing recipient serum against donor red cells
c) Comparing blood groups by genetic analysis
d) Screening for infectious agents - The immediate-spin crossmatch is primarily used to detect:
a) ABO incompatibility
b) Rh typing errors
c) IgG antibodies
d) Enzyme-sensitive antibodies - The indirect antiglobulin crossmatch is used to detect:
a) Cold agglutinins
b) IgM antibodies only
c) Clinically significant IgG antibodies
d) ABO grouping discrepancies - The electronic crossmatch is allowed when:
a) Antibody screen is positive
b) Patient has clinically significant antibodies
c) No unexpected antibodies are detected
d) Donor and recipient are different Rh types - A major crossmatch involves testing:
a) Donor serum with donor cells
b) Recipient serum with donor cells
c) Donor serum with recipient cells
d) Recipient plasma with their own cells - A minor crossmatch involves testing:
a) Donor plasma with recipient red cells
b) Donor serum with donor cells
c) Recipient serum with donor cells
d) Serum from both donor and recipient together - Which anticoagulant is most commonly used in blood collection bags for crossmatch samples?
a) EDTA
b) Citrate phosphate dextrose (CPD)
c) Heparin
d) Sodium fluoride - Crossmatching must always be performed using:
a) Fresh donor samples only
b) Patient plasma or serum
c) Saline solution
d) Red cell stroma - What is the valid time limit for pretransfusion patient samples if transfused or pregnant within the past 3 months?
a) 48 hours
b) 72 hours
c) 7 days
d) 30 days - In an emergency transfusion, uncrossmatched group O blood is given when:
a) Blood typing is incomplete
b) Patient has known antibodies
c) Only Rh-negative units are available
d) Platelet counts are low - Which type of incompatibility causes the most severe transfusion reactions?
a) ABO incompatibility
b) Rh mismatch
c) Kell antibody reaction
d) Kidd antibody reaction - A positive crossmatch result indicates:
a) Compatibility between donor and recipient
b) Presence of an antibody reacting with donor cells
c) Lack of antibody detection
d) Technical error only - In the antiglobulin crossmatch, AHG reagent is added to:
a) Detect IgG antibodies bound to red cells
b) Detect complement in plasma
c) Remove unbound antibodies
d) Inactivate antibodies - A unit labeled as O positive can be safely transfused to:
a) Any recipient
b) Only O positive individuals
c) AB positive individuals in emergencies
d) A negative individuals - Which step must always be completed before issuing blood for transfusion?
a) Crossmatching
b) Plasma glucose testing
c) Platelet count
d) Coagulation screening - What does a negative crossmatch indicate?
a) No incompatibility detected
b) Weak antibody present
c) Invalid test result
d) Donor blood unsuitable - Which of the following errors can cause false-positive crossmatch results?
a) Dirty glassware or contaminated reagents
b) Cold autoantibody interference
c) Incorrect temperature
d) All of the above - The purpose of the antibody screen before crossmatch is to:
a) Detect unexpected antibodies in patient serum
b) Identify donor blood group
c) Confirm ABO compatibility
d) Detect Rh status - The pretransfusion testing procedure includes:
a) ABO, Rh typing, antibody screen, and crossmatch
b) Hemoglobin and hematocrit testing only
c) White cell count and morphology
d) DAT and ESR - The Rh type of a patient should be confirmed using:
a) Weak D testing if initial test is negative
b) Direct antiglobulin test
c) Auto control
d) Enzyme test only - What is the minimum number of patient identifiers required for sample labeling?
a) One
b) Two
c) Three
d) Four - When is compatibility testing not required?
a) When issuing plasma products
b) When transfusing whole blood
c) When patient has positive antibody screen
d) When transfusing red cells - Which phase of testing is skipped during electronic crossmatch?
a) Immediate-spin
b) 37°C incubation
c) AHG phase
d) All physical testing phases - The recipient’s sample must be tested against:
a) Donor’s red cells
b) Donor’s plasma
c) Another patient’s cells for control
d) Reagent red cells only - Clerical errors in crossmatching most commonly involve:
a) Misidentification of patient or sample
b) Incorrect reagent preparation
c) Contaminated saline
d) Instrument malfunction - Compatibility testing is not designed to detect:
a) All clinically significant antibodies
b) Infectious disease markers
c) ABO incompatibility
d) Rh mismatch - In emergency transfusions with unknown blood type, which type is preferred for females of childbearing age?
a) O positive
b) O negative
c) A positive
d) B negative - A unit that is “crossmatch compatible” means:
a) No agglutination occurred between donor cells and recipient serum
b) Strong positive reaction was observed
c) The donor sample is antigen-positive
d) The patient has no antibodies at all
📌 How to Use This Practice Set
- Answer each question before checking the key.
- Focus on why the correct answer is right and the others are wrong.
- Use this set as timed practice to simulate the real exam environment.
Answer Key
Answer Key:
- c) To detect recipient antibodies directed against antigens on donor red cells
- a) All major crossmatches would be incompatible
- d) Donor red cells have a positive direct antiglobulin test
- a) The patient has a historical ABO and Rh in the computer system
- b) Discard the unit as temperature was not maintained
- c) Anti-A; anti-B; A1 cells; B cells
- b) Detect ABO incompatibility between donor and recipient
- c) Friday at 11:59 PM
- b) A statement that compatibility testing has not been completed
- d) O, Rh-negative
- d) O, Rh-negative
- a) Phenotype his cells to determine which alloantibodies he can produce
- d) False-negative due to antigen excess
- d) Donation identification number and donor’s ABO and Rh
- a) Recipient alloantibody
- c) Perform an elution and identify the antibody in the eluate
- d) ABO forward typing and Rh testing for D antigen
- c) Date blood typing was performed
- b) Fresh Frozen Plasma and Apheresis Platelets
- d) Phenotype donor units and select Jk(b–) units for compatibility testing
- d) Group AB, Rh-positive
- a) Light spin followed by a hard spin
- c) Reduced by 14 days
- b) 24 hours
- c) The platelet count is the expected posttransfusion increment
- d) 30,000-50,000/µL
- b) An IgA-deficient patient with a history of anaphylaxis
- b) 9.0 g/dL
- b) Diversion pouch
- a) Match the blood component to the recipient using 2 independent identifiers
- b) Detect antibodies that could cause transfusion reactions
- c) ABO and Rh typing
- b) Testing recipient serum against donor red cells
- a) ABO incompatibility
- c) Clinically significant IgG antibodies
- c) No unexpected antibodies are detected
- b) Recipient serum with donor cells
- a) Donor plasma with recipient red cells
- a) EDTA
- b) Patient plasma or serum
- b) 72 hours
- a) Blood typing is incomplete
- a) ABO incompatibility
- b) Presence of an antibody reacting with donor cells
- a) Detect IgG antibodies bound to red cells
- b) Only O positive individuals
- a) Crossmatching
- a) No incompatibility detected
- d) All of the above
- a) Detect unexpected antibodies in patient serum
- a) ABO, Rh typing, antibody screen, and crossmatch
- a) Weak D testing if initial test is negative
- b) Two
- a) When issuing plasma products
- d) All physical testing phases
- a) Donor’s red cells
- a) Misidentification of patient or sample\
- b) Infectious disease markers
- b) O negative
- a) No agglutination occurred between donor cells and recipient serum
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Top 8 Medical Laboratory Scientist (MLS) Exams that are recognized globally and can help professionals validate their credentials and enhance their career opportunities:
1. ASCP – American Society for Clinical Pathology (USA)
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