Pretransfusion Compatibility Testing
Core testing
Pretransfusion testing comprises patient identification and specimen labeling, ABO/Rh typing with forward and reverse grouping, antibody screen, and crossmatch. Immediate-spin crossmatch verifies ABO compatibility only; an AHG crossmatch detects IgG incompatibility and is required when the screen is positive or a clinically significant antibody history exists. The antibody screen, not the crossmatch, is the principal safety test; a compatible crossmatch does not guarantee normal in-vivo survival, particularly with Kidd or Duffy antibodies.
Electronic crossmatch
Permitted only when: two concordant ABO determinations exist (current specimen plus a historical or second specimen), there is no ABO discrepancy, the antibody screen is negative, there is no history of clinically significant antibodies, the LIS has been validated with a bidirectional interface, units carry ISBT 128 labels, and the medical director has approved the policy. Never use an electronic crossmatch when the screen is positive.
Special situations
- Emergency: uncrossmatched group O red cells, O-negative for females of childbearing potential and O-positive acceptable for others per policy; never delay transfusion for compatibility testing. Switch to the patient's ABO type once known.
- Alloimmunized patients: antigen-negative units with AHG crossmatch. Least incompatible units require physician sign-off and documentation.
- Neonates: maternal antibody screen and, if positive, AHG crossmatch using maternal serum; a maternal sample may be used for up to 3 months.
- Special products: irradiated (25-50 Gy), CMV-safe (leukoreduced), washed for IgA deficiency or severe anaphylaxis, volume-reduced, and leukoreduced.
Traps
Misidentified or diluted specimens (wrong-patient draws are the leading cause of fatal hemolytic reactions), unavailable historical records, and confusing a type and screen with a type and crossmatch. D-negative recipients receive D-negative units unless policy permits otherwise.
Worked example: a group A, D-negative patient with anti-K receives A-negative, K-negative units that are AHG crossmatch compatible; group O units would work but waste a scarce resource.