Serological crossmatch procedure. The crossmatch is the final step of pretransfusion compatibility testing. It serves 1) as a final check of ABO compatibility between donor red blood cells (RBCs) and patient plasma or serum and 2) to detect clinically significant antibodies directed against antigens on the donor RBCs that may have been missed by the antibody screening test. Patient plasma is mixed with donor cells. The sample is centrifuged at room temperature and examined for hemolysis or macroscopic agglutination either immediately (immediate spin [IS] crossmatch) or following incubation with anti-human globulin (AHG) (the AHG crossmatch). Hemolysis or agglutination of the donor cells is indicative of an antibody in the recipient’s serum that is directed against an antigen on the donor cells, hence an incompatible crossmatch. In that case, the donor unit is deemed incompatible for transfusion, and further investigations are undertaken to identify the antibody responsible for the reaction. No agglutination can be interpreted as a compatible crossmatch if the patient has a negative antibody screen.
