Isoagglutinin dynamics and transfusion dependency after ABO‐ incompatible haematopoietic stem cell transplants

BACKGROUND: ABO incompatibility is common in allogeneic haematopoietic stem cell transplantation (HSCT) and may delay erythroid recovery and prolong transfusion support. Data correlating post-transplant isoagglutinin kinetics with transfusion dependency is limited. METHODS: We retrospectively analysed ABO-incompatible allogeneic HSCTs performed between June 2008 and December 2023. Patients were classified as major, minor or bidirectional incompatibility. Serial Immunoglobulin M (IgM) and Immunoglobulin G (IgG) anti-A and anti-B isoagglutinin titres were monitored post-transplant. Time to isoagglutinin disappearance, donor red blood cell (RBC) appearance and associations with packed red blood cell (PRBC) and single donor platelet (SDP) transfusion requirements were evaluated. RESULTS: Among 459 allogeneic HSCTs, 183 (39.9%) were ABO-incompatible. Median isoagglutinin disappearance was 28 days in major and 38.5 days in bidirectional incompatibility, with delayed donor RBC appearance in the latter. Anti-A isoagglutinins persisted longer than anti-B. Persistent isoagglutinins correlated with increased early PRBC and SDP transfusion requirements. CONCLUSIONS: Isoagglutinin kinetics vary by ABO incompatibility type and are associated with early transfusion dependency after HSCT.