Diagnosis of Acute Hemolytic Transfusion Reaction Caused by Cold Reactive Anti‐P1 Antibody: A Case Report

Anti‐P1 alloantibody is predominantly an immunoglobulin M class, cold reactive antibody that is non‐reactive at 37°C in routine pre‐transfusion serology and thus generally regarded as clinically insignificant. However, rare clinically significant anti‐P1 antibodies can cause an acute hemolytic transfusion reaction (AHTR), and routine pre‐transfusion testing at 37°C may miss such antibodies. We report a case of AHTR caused by an anti‐P1 antibody detectable only at 4°C, which evaded detection by unexpected antibody screening performed at 37°C. A 65‐year‐old woman with blood group B, RhD positive developed AHTR within 24 h following red blood cell transfusion. She presented with coffee‐colored urine, markedly elevated bilirubin and lactate dehydrogenase concentrations, and a progressive decline in hemoglobin concentrations. Pre‐transfusion antibody screening at 37°C was negative. A retrospective serological analysis identified anti‐P1 exclusively at 4°C, which was confirmed by pigeon egg white inhibition. The patient was P1 negative; incompatibility occurred only with the P1‐positive donor unit. After transfusion of P1 antigen‐negative red blood cells, the hemolysis resolved and hemoglobin concentrations recovered. In patients with a repeated transfusion history, serological testing at 37°C and 4°C, in conjunction with antigen‐matched selection, is essential to prevent AHTR caused by clinically significant cold reactive antibodies.