The ability to identify blood group antigens in a potential transfusion recipient is limited by the availability of reagents for each antigen for each species. Reagents are available for only some antigens, generally those that are most likely to sensitize a recipient, or those for which naturally occurring antibodies might be present. For example, dogs have more than 12 blood group systems but generally are typed for only one (DEA 1.1). Recently an additional blood group antigen (dal) was discovered when a dal-negative Dalmatian reacted to many potential donors, and only a few Dalmatians were found to be compatible. It is a common antigen in most dogs but is lacking in some Dalmatians. Because multiple blood group antigens are present, it is likely that an animal receiving a transfusion might be exposed to some antigens that are not present on its RBC.

Previously sensitized recipients can be detected by crossmatching, which is done to preclude administration of incompatible blood. In the USA, >99% of cats are of blood group A, so the risk of incompatible transfusion is low. However, certain breeds, including Abyssinian, Birman, British Shorthair, Devon Rex, Himalayan, Persian, Scottish Fold, and Somali, have a higher frequency of blood group B. Any incompatible transfusion in cats results in rapid destruction of transfused cells, so typing and crossmatching should be done before any transfusion. A newly discovered antigen in cats (mic) is present in some cats, and naturally occurring antibodies are present in cats lacking the mic antigen. For that reason, crossmatching should be performed for cats before the first transfusion, even if they will receive A or B matched blood.

The direct crossmatch procedure, with appropriate controls, is effective for all species. The major crossmatch detects antibodies already present in recipient plasma that could cause a hemolytic reaction when donor RBC are transfused; it will not detect the potential for sensitization to develop. Anticoagulant (calcium disodium edetate or citrate) is added to blood samples from donor and recipient; the donor RBC are washed 3 times with 0.9% saline, and a 4% RBC suspension in saline is made from the washed cells. The major crossmatch consists of combining equal volumes (0.1 mL) of the donor RBC suspension and recipient plasma. The control tube contains recipient RBC and recipient plasma. The samples are incubated, centrifuged, and evaluated for hemolysis or agglutination. Hemolysis is evaluated by comparing the color of the supernatant in the test sample with that of the control sample. Each sample is then gently shaken until all cells in the “button” at the bottom of the tube have returned to suspension. Again, the degree of cell clumping of the test sample is compared with that of the control sample. The test is negative or compatible when the plasma is clear and the RBC are readily suspended. A positive or incompatible test can have hemolysis or hemagglutination, or both. All tests judged macroscopically to be negative for hemagglutination should be confirmed microscopically at low power. Some newer crossmatching systems using a gel technique are becoming available. This is particularly important in horses because their RBC tend to form rouleaux.

The minor crossmatch is the reverse of the major crossmatch, ie, recipient cells are combined with donor plasma. The minor crossmatch is important only in species such as cats with clinically significant naturally occurring isoantibodies or if the donor has been previously transfused or, in horses, previously pregnant.

Last full review/revision July 2011 by Susan M. Cotter, DVM, DACVIM (Small Animal, Oncology)