Wikipedia
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.
Cross-matching is done by a certified laboratory technologist, in a laboratory. It can be done electronically, with a computer database if a patient has previously been tested, or serologically by way of testing. Simpler tests may be used to determine blood type (only), or to screen for antibodies (only). ( indirect Coombs test).