A crossmatch is performed prior to administration of blood or blood products (e.g. packed red blood cells). The purpose of the crossmatch is to detect the presence of antibodies in the recipient against the red blood cells of the donor. These antibodies attach to the red blood cells of the donor after transfusion. An incompatible transfusion can result in a severe hemolytic anemia and even death. In dogs and horses, naturally occurring antibody against important hemolytic red blood cell antigens (e.g. DEA 1.1 and 1.2 in the dog, and Qa and Aa in the horse) are not found. Therefore, these animals require sensitization to the red cell antigen, before a hemolytic reaction will occur. This sensitization usually occurs from a previous blood transfusion. Therefore, in these species, a crossmatch is not absolutely required on the first blood transfusion (as long as you are sure this is the first transfusion). Once a blood transfusion has been administered to a dog or horse, a crossmatch should be performed prior to any subsequent transfusions to detect antibodies that may have been produced against a different red blood cell antigen. In cats, a crossmatch should be performed on the first blood transfusion, because cats have naturally occurring antibody to red blood cell antigens. In type B cats, the anti-A antibody is a strong agglutinin and hemolysin and can result in rapid hemolytic anemia and death if a B cat is transfused with A blood on the first transfusion. Type B cats are uncommon amongst DSH, but are found in higher frequency amongst the exotic breeds, e.g. Somali, Devon Rex.

For a crossmatch procedure,we do 3 types of crossmatches:

• Major crossmatch: This is the most important one. In this procedure, we are looking for antibodies in the recipient against transfused red blood cell antigens (from the donor). Therefore, we need serum from the recipient and red blood cells from the donor.
• Minor crossmatch: This detects antibodies in the donor serum to the recipient's red blood cells. Therefore, for this we need serum from the donor and red blood cells from the recipient.
• Autocontrol: We also perform an auto-control with our crossmatches, i.e. recipient serum with recipient red blood cells.

In these procedures, washed red blood cells are incubated with serum at 37 C (e.g. for the major crossmatch, washed donor red blood cells are incubated with recipient serum). We then look for agglutination microscopically. In horses, we add complement (to enhance hemolysis) and look for both microscopic agglutination and grossly visible hemolysis. In horses, we also perform the test at 2 dilutions, 1:4 and 1:16.

We use the following guidelines for interpretation of the crossmatch:

In horses, an incompatible crossmatch with a titer > 1:16 should not be administered. A weak incompatible reaction (titer <1:16) may be due to anti-Ca antibodies, which will not result in a hemolytic reaction. However, the titer at which hemolysis may be expected has only been established for anti-Aa and anti-Qa antibodies. Although these are the most common red cell antigens that cause transfusion reaction in horses, reactions to other red cell antigens (e.g. Pa) have been reported. The titer at which these antibodies are likely to produce a reaction is not known. For this reason, unless the blood type of the donor and recipient horse are known, we recommend that a crossmatch with a weak positive titer should not be administered.