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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 Québec (Canada). Transfusion safety officers reported laboratory AHTR indicators measured ≤24 h pre- and post-transfusion. Because the AHTR case definition of Québec'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., (© 2023 International Society of Blood Transfusion.)

Background and Objectives: A high proportion of suspected weak D patients referred to Héma-Québec were genotyped as weak D type 42 (368/2105, 17.5%). These patients are currently considered D with regard to RhD immunoprophylaxis in pregnancy and transfusion. The goal of this study was to retrospectively evaluate the risk of alloimmunization in weak D type 42 patients and to characterize their RhD surface molecule expression on red blood cells (RBCs) in comparison to other weak D types (1, 2 and 3)., Materials and Methods: A retrospective analysis using the weak D type 42 patients' medical data to verify potential anti-D alloimmunization events was conducted. Quantitative analyses using flow cytometry were also performed on RBCs to quantify the cell surface density of the D antigen., Results: Data on 215 subjects with weak D type 42 were reviewed. None developed immune allo-anti-D; three had definite exposure to D+ red cells and 41 had possible exposure through pregnancy. Flow cytometry analysis showed that weak D types 1, 2, 3 and 42 had relative antigen densities of 2.7%, 2.2%, 8.1% and 3.6%, respectively, with R1R2 red cells referencing 100% density. The estimated antigen density range of weak D type 42 was 819-1104 sites per RBC., Conclusion: Our retrospective alloimmunization data analysis and antigen density study establish a basis for the consideration of a weak D type 42 individual as D+. This consideration would allow for a targeted reduction of RhD immunoprophylaxis in pregnancy and the unjustified use of D- units for transfusion., (© 2022 International Society of Blood Transfusion.)