ABO antigen (Ag) is expressed not only on red blood cells (RBCs), but also on parenchymal organs such as liver, spleen, and kidneys. Therefore, it is necessary to perform ABO antibody (Ab) titration tests for the assessment of prognosis in ABO-incompatible hematopoietic stem cell transplantation or for decision-making in ABO-incompatible kidney transplantation. Despite such practical importance, there are inter-laboratory variations in the measurement of the ABO Ab titer due to the availability of various different techniques and the absence of a standard method.