Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
In transfusion medicine, cross-matching refers to the testing that is performed to determine the compatibility of a donated unit of blood for its intended recipient. It should not be confused with tests to determine a blood type or an antibody screen (indirect Coombs test) which are two entirely different tests that should always be completed prior to cross-match testing. Cross-match testing is done by a qualified laboratory technologist in a blood bank. Cross-matching can be done electronically with a computer database or serologically.
An electronic cross-match is essentially a computer assisted analysis of the data entered from testing done on the donor unit and blood samples drawn from intended recipient. This includes ABO/Rh typing of the unit and the recipient and an antibody screen of the recipient. Electronic cross-matching can only be used if a patient has a negative antibody screen. This means that they do not have any active red blood cell atypical antibodies or they are below the detectable level of current testing methods. If all of the data entered is compatible the computer will print a compatibility label stating that the unit is safe to transfuse.
In a serological cross-match red blood cells from the donor unit are tested against the plasma/serum of the patient who is in need of the blood transfusion. If the patient’s serum contains antibodies against the antigens present on the donor red blood cells agglutination will occur. Agglutination is considered a positive reaction indicating that the donor unit is incompatible for that specific patient. If no agglutination occurs the unit is deemed compatible and is safe to transfuse.
In the case of an emergency a physician can request uncross-matched blood, it is thought that the lifesaving measure is of more benefit than any risk of an antibody mediated transfusion reaction; this blood must be ABO compatible and preferably Rhesus (Rh) compatible as well to minimize the risk of a serious transfusion reaction. Type O Rh negative blood can be given if the recipients blood group is not known, as may happen in an emergency. In an emergency, blood grouping can be done easily and quickly in 2 or 3 minutes in the laboratory on glass slides with appropriate reagents, by trained technical staff. These slide methods depend on the presence or absence of agglutination that can usually be visualized directly, but occasionally a light microscope may be needed. If laboratory services are not available, another system of deciding which type of blood to use in an emergency is the bedside card method of blood grouping, where a drop of the intended recipients' blood is added to dried reagents on a prepared card. The card method may not be as reliable as laboratory methods, which are preferable.
Cross matching is also frequently used in determining good candidates for organ transplants.
- HealthAtoZ.com Blood typing and crossmatching
- Nobelprize.org Interactive online game for blood typing and transfusion (Flash Player 5 required)