RBC Crossmatch
The crossmatch is the final step in confirming the compatibility of RBCs selected for transfusion, and serves as a final check against the accuracy of the patient’s preceding type and screen.
To confirm ABO compatibility, the patient’s plasma should be shown to not directly agglutinate the donor RBCs. Mixing plasma and RBCs together and then centrifuging briefly is adequate for this purpose and is referred to as an abbreviated or immediate spin crossmatch; 15 minutes is typically all that is required, although the preceding type and screen may take an hour to complete. Blood bank computer systems can also apply Landsteiner’s Law to confirm ABO compatibility without any serologic testing at all, once the ABO type of the donor and recipient have been confirmed. This is known as an electronic crossmatch and can be performed in seconds.
To confirm lack of incompatibility due to minor blood type antibodies as well as ABO, however, patient plasma and donor RBCs must be incubated together in the presence of antiglobulin reagent; this is referred to as a full serologic crossmatch. Because it takes longer to perform (approximately one hour) it is usually reserved for patients who have already had minor blood type antibodies detected. When performing a full crossmatch, it is necessary to select RBCs that have been shown to be negative for antigens which the patient has developed antibodies to; if the crossmatch is nonetheless incompatible, this may mean that the type and screen (and associated antibody investigation, if indicated) was not performed properly and should be repeated.
Other reasons for unexpectedly positive crossmatches include the presence of incompatibility to minor blood type antigens that were not expressed on the cells used for antibody screening, the presence of an autoantibody in the plasma of either the patient or the blood donor, or clerical error.
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