Cross-matching test, which is a concurrent agglutination test of the recipient’s serum plus the donor’s red blood cell suspension and the donor’s serum plus the recipient’s red blood cell suspension. The crossmatch test consists of two types of tests: the primary test and the secondary test. The former test is done with the recipient’s serum and the donor’s red blood cell suspension to find out whether the recipient’s serum contains antibodies that react with the donor’s red blood cells, also known as the direct match or primary match. In the latter case, the donor’s serum is tested against the recipient’s red blood cells to find out whether the donor’s serum contains antibodies that do not react with the donor’s red blood cells, also known as an indirect match. Its purpose is to verify whether the donor and recipient ABO blood group identification is correct, in order to prevent hemolytic transfusion reaction. In addition, irregular agglutinins of the ABO blood group system can also be detected, as well as the discovery of antibodies to blood groups other than the ABO system. There are various ways to perform the crossmatch test, such as saline method, colloidal media matching method, antiglobulin method, and polyglutamine method. When transfusing blood, not only the ABO blood group and Rh blood group must be identified, but even if the same type of blood is entered, or if the recipient receives blood from the same donor again, a cross-matching test must be done.