ABO serology in a case of persistent weak A in a recipient following a group O-matched unrelated bone marrow...

HLA-matched hematopoietic stem cell transplantation (HSCT) from red blood cell (RBC)-incompatible donors is not uncommon. The engraftment process following ABO-incompatible allogeneic HSCT results in the transition from patient blood group to donor blood group in the recipient. In contrast, most non-hematopoietic tissues retain expression of the patient's original blood group for life, and these antigens may adsorb from the plasma onto the donor-derived RBCs. Correct serologic interpretation of the ABO blood group during this engraftment process can be difficult. We present the serologic findings of a 15-year-old girl of Maori descent, who was diagnosed with acute myeloid leukemia and transplanted with an HLA-matched unrelated group O, D+ bone marrow. Despite engraftment, her RBCs showed persistence of weak A. This case report showcases the importance of awareness and correct serologic interpretation of weak persistence of recipient ABH substance on the patient's RBCs for clinical decision-making, blood component support, and patient wellbeing.