Incidence of acute haemolytic transfusion reaction among ABO-incompatible recipients transfused with A₃ blood: A case series

Background and Objectives: ABO antigens are among the most immunogenic, but the haemolytic risks of ABO incompatibilities involving a donor with a weak ABO phenotype are little documented. Materials and Methods: This retrospective case series assessed the incidence of acute haemolytic transfusion reaction (AHTR) among ABO-incompatible recipients of A(3) blood in Quebec (Canada). Transfusion safety officers reported laboratory AHTR indicators measured <= 24 h pre- and post-transfusion. Because the AHTR case definition of Quebec's Hemovigilance System (QHS) leaves significant room for clinical judgement, a two-step approach was used to assess potential cases: Step 1 consisted in a highly sensitive-but unspecific-initial screen that identified all candidate cases per QHS case definition, and Step 2 consisted in a detailed review of candidate cases by two haematologists. Results: Nine donors initially typed as Group B (N = 1) or O (N = 8) were subsequently found to display an A(3)B or A(3)O phenotype. Eighty-one recipients received ABO-incompatible blood, including 53 (65.4%) with interpretable data. Of these, 29 (54.7%) were classified as candidate cases after Step 1. Following Step 2, no conclusive evidence of AHTR was found: Abnormal pre- versus post-transfusion changes appeared modest, within normal range, insufficient to ascertain AHTR, or were consistent with a pre-existing condition unrelated to AHTR. Two candidate cases had a QHS-reported transfusion reaction; both were unrelated to AHTR. Conclusion: In this case series, no conclusive evidence of serious AHTR was found among ABO-incompatible recipients who were inadvertently transfused with A(3) blood.