Previously sensitized recipients or those with naturally occurring antibodies 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. The mic antigen is present in some cats, and naturally occurring antibodies are present in cats that lack 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 RBCs 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 RBCs 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 RBCs 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 RBCs 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 that use a gel technique are becoming available. This is particularly important in horses, because their RBCs 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, those with previous pregnancies.