366 The Not So Insignificant Anti-A1 Antibody: Cause of Severe Hemolytic Transfusion Reaction

Anti-A1 antibody found in A2 and A2B patients binds A1 cells. In most cases, anti-A1 is of no clinical significance, reacting well below body temperature, and is merely a laboratory nuisance causing ABO discrepancies. When anti-A1 is active at body temperature, though rare, extensive destruction of A1 cells in vivo can occur and has been documented. Here we report a case of a severe hemolytic transfusion reaction in an A2 patient after transfusion of A1 red cells. Transfusion of three A1 RBC units resulted in a hemoglobin drop from 9.4 to 6.4 g/dL in 48 hours with no identifiable surviving A1 cells, suggesting complete destruction of transfused cells. Post-transfusion sample showed marked icterus, ABO discrepancy in reverse typing, and a negative antibody screen. IRL workup confirmed patient is negative for A1 antigen and has significant anti-A1 titers. Patient successfully received two units of group O RBCs with expected increased in hemoglobin that was sustained. Approximately 80% of individuals in the US with blood group A express A1. Anti-A1 occurs in 1%-8% of A2 individuals and 22%-35% of A2B individuals. A2 patients with an anti-A1 reactive at 37°C should be transfused with group O or A2 RBCs only. A2B patients should receive group O, A2, A2B, or B RBCs. It is not current practice to test A or B cells as part of a routine eluate panel, though one could consider adding them to routine testing methods. Our case highlights that anti-A1 can cause severe hemolytic anemia. This has important implications for transplants because while successful renal transplantation has been reported across the A1/A2 blood group barrier in the setting of low titer anti-A1 isoagglutinins, more aggressive anti-A1 titer monitoring along with hemolysis labs may be warranted to assess possibility of clinically significant hemolysis.