Uncommon postpartum delayed hemolytic transfusion reaction caused by anti-C antibodies: A rare presentation without prior risk factors.

Delayed hemolytic transfusion reaction (DHTR) is a rare post-transfusion complication, typically involving extravascular hemolysis. We present an unusual case of postpartum DHTR caused by anti-C antibodies in a 35-year-old Japanese woman, gravida 2, para 1, with no history of transfusion, alloimmunization, or underlying hemoglobinopathy. The patient developed dark-colored urine on postpartum day 25, 12 days after receiving a blood transfusion for hemorrhage due to placenta accreta spectrum. Laboratory findings revealed indirect hyperbilirubinemia, elevated lactate dehydrogenase, low haptoglobin, and hemoglobinuria, indicating concurrent extravascular and intravascular hemolysis. DHTR was confirmed by the detection of anti-C antibodies and the presence of C-antigen-positive transfused red cells. Maternal sensitization likely resulted from subclinical fetomaternal transfusion, with an immune response triggered post-transfusion. This case highlights the diagnostic challenges of DHTR in the postpartum setting, emphasizing the importance of post-transfusion antibody screening and the need for heightened awareness of unexplained fever as a potential early indicator.