BACKGROUND: The ABO antibody titration is important, especially in case of ABO-incompatible hemolytic disease of newborn, ABO-incompatible bone marrow or solid organ transplantation. However, no standard method for ABO antibody titration has yet been established. We surveyed four university hospitals about the methods of ABO antibody titration and performed inter-laboratory proficiency tests. METHODS: Detailed methods of ABO antibody titration were surveyed at four university hospitals. ABO antibody titer was measured by their customary methods using serum samples from six healthy volunteers with blood groups A (n=2), B (n=2) and O (n=2). RESULTS: Procedures of ABO antibody titration, reportable ranges, sample diluent, source of reagent RBCs and interpretation of end-point were different among four university hospitals. Inter-institutional maximum differences of IgM and IgG ABO antibody titer were 16-fold and 32-fold, respectively. CONCLUSION: Standardization of ABO antibody titration method is needed to reduce inter-laboratory variability, and a periodical external quality control survey is necessary to improve the accuracy of the titrationope
