Recognizing and resolving ABO discrepancies

Patient samples are routinely typed for ABO prior to transfusion. Determining the ABO group requires both red blood cell (RBC) antigen typing for A and B (forward type) and testing for anti-A and anti-B in the plasma (reverse type). An ABO discrepancy exists when the result of an ABO RBC typing, or forward type, does not agree with the result of the plasma typing, or reverse type. This brief review examines several causes of ABO discrepancies encountered in the clinical transfusion service. Options for resolving these discrepancies are presented, including a discussion of which discrepancies should be resolved using molecular testing. Finally, case studies illustrate transfusion options in patients with ABO discrepancies. Discrepancies can also be encountered when ABO typing is performed on samples from blood or tissue donors, although those discrepancies will not be discussed in this review.