Delayed hemolytic transfusion reaction caused by anti-P1 antibody

A 67-year-old white woman received transfusions of a total of 87 units of whole blood and red blood cells during and within 48 hours following a pneumonectomy. Although she had previously received blood transfusions, unexpected antibodies were not detectable by routine screening. On the second postoperative day, she developed fever, hemoglobinemia, hemoglobinuria, and oliguria. However, the direct antiglobulin test and the antibody screen were negative. On the eighth postoperative day, an IgM anti-P1 antibody was detected for the first time. This anti-P1 antibody increased in thermal amplitude from 22 to 37 C, but remained IgM. The circulating transfused P1-positive cells decreased progressively without evidence of bleeding. Testing of the patient's preoperative blood at 15 C found her serum to be weakly reactive with P1 cells, while her own cells were P2. Thus, an anamnestic response to the P1 antigen is the most likely cause of her delayed hemolytic transfusion reaction.