The impact of ABO compatibility on allogeneic hematopoietic cell transplantation outcomes: a contemporary and comprehensive study from the transplant complications working party of the EBMT.

The role of ABO blood group system mismatch on allogeneic hematopoietic cell transplantation (allo-HCT) outcomes is controversial since current publications of large datasets are lacking. We retrospectively analyzed 30,487 patients transplanted between 2010 and 2021 using the EBMT registry to assess ABO incompatibility's effect on non-relapse mortality (NRM), overall survival (OS), progression-free survival (PFS), relapse incidence (RI), acute GvHD (aGvHD), chronic GvHD (cGvHD), and neutrophil engraftment. Transplantations were classified as ABO-compatible (56.3%), major (18.1%), minor (20.1%), and bidirectional (5.5%) incompatibilities. Mainly peripheral blood stem cells (PBSC) were used as the cell source in 85.6% of cases. Multivariate analysis found no significant association between compatibility status, with the compatible group serving as the reference, and NRM, OS, PFS, RI or cGvHD. The incidence of non-engraftment was significantly higher in the major (HR 1.04, 95% CI 1.01-1.07, p = 0.021) and bidirectional (HR 1.09, 95% CI 1.03-1.15, p = 0.003) incompatibilities. At the same time, the risk of severe aGvHD grades III-IV was lower in the major incompatibility group (HR 0.85, 95% CI 0.77-0.94, p = 0.001). Our large contemporary study, showing no major impact on outcomes, suggests that the ABO blood group system should not be a primary consideration in donor selection for PBSC-based allo-HCT.