The tube test is the most commonly performed method in laboratories. Although in the tube test, a manual reading with the naked eyes is required for grading the agglutination strength and automation of the test is difficult, it is being widely used for general ABO Ab titration in Korea. For the gel card test, one of the column agglutination techniques (CAT), a commercially prepared card with microcolumns containing gel particles is used. The gel card test is more qualitative in grading the strength of agglutination reaction. Additionally, it is less time-consuming and uses smaller volumes of serum and RBCs, and can be used as part of an automated system. Therefore, the number of laboratories using CAT as a method for ABO Ab titration has increased in recent years, despite the need for expensive reagents. Recently, application of flow cytometry (FCM) was suggested as a sensitive ABO Ab detection method [5, 6, 7, 8], and was reported to accurately measure IgM and IgG by the use of isotype-specific Ab. Some studies have reported that FCM produces relatively comparable results to those of the microplate HA assay [5] and CAT [8]. While FCM has been known as a more sensitive method than an agglutination method for Ab detection such as the HLA Ab, the evidence remains too sparse to draw a conclusive evaluation for detecting ABO Ab.