Cross-matching (or crossmatching) blood, in transfusion medicine, refers to the test that is performed prior to a blood transfusion in order to determine if the donor's blood is compatible with the blood of an intended recipient. Cross-matching is also used to determine compatibility between a donor and recipient, in organ transplantation or blood transfusion. Compatibility is determined through matching of different blood group systems, the most important of which are the ABO and Rh system, and/or by directly testing for the presence of antibodies against a sample of donor tissues or blood.