A discrepancy in ABO typing occurs when the front and back types do not agree. Resolving the discrepancy requires an evaluation of both the front and back typings.
Front type: testing the patient's red blood cells with reagent antisera
Causes of unexpected positive reactions:
(1) chimera following bone marrow transplant
(2) polyagglutinable red blood cells
(3) acquired A or B substance
(4) recent transfusion
(5) red blood cells coated with protein (rouleaux, etc), often due to inadequate washing
(6) autoantibody
Causes of unexpected negative reactions:
(1) decreased antigen expression on the red blood cells
(2) inactivated antisera
(3) masked antigenic sites due to protein or autoantibody
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