The crossmatch procedure determines whether donor blood is compatible (or incompatible) with recipient blood. A crossmatch should always be performed in the following situations:
- An animal of a species which contains naturally occurring pathogenic antibodies to foreign blood group antigens. This occurs in the cat. In this species, a crossmatch should be performed on the first and every transfusion, unless the blood types of both donor and recipient are known. In cat breeds in which there is a low percentage of type B cats (e.g. Siamese, DSH or DLH in the USA), transfusion of blood from an uncrossmatched or untyped donor can be performed relatively safely. However, there is still a (albeit small) chance of a major transfusion reaction and this procedure is not recommended.
- An animal of a species which does not contain naturally occurring antibodies but has been sensitized to foreign red cell antigens, with the production of acquired antibodies. This is the situation in the dog and horse. In these species, a crossmatch does not need to be performed on the first transfusion the animal receives (as long as you can be sure this is the first transfusion ever), but should be performed at subsequent transfusions (if the interval between the first and subsequent transfusions is more than 5 days).
Crossmatching will pick up incompatibilities between the donor and recipient that will not be evident on blood typing (as blood typing is not available against every blood group, just the major ones). In addition, the crossmatch procedure will not pick up low titer antibodies and thus will not prevent delayed-type hemolytic transfusion reactions (for more information, see adverse reactions).
There are two types of crossmatches:
- Major crossmatch: This is the most important crossmatch, comparing donor erythrocytes to recipient serum (i.e. you are checking for preformed (acquired or naturally occurring) antibodies in recipient serum against donor erythrocytes. For the major crossmatch, you need red blood cells from the donor (this can be whole blood from a donor animal or packed red blood cells) in EDTA or citrate and serum from the recipient (non-anticoagulant tube).
- Minor crossmatch: This compares donor serum to recipient erythrocytes and checks for preformed antibodies in donor serum that could hemolyse recipient red cells. This crossmatch is less important as usually the donor serum is markedly diluted after transfusion and is unlikely to produce a significant transfusion reaction. This type of crossmatch could be important if transfusing small patients, in which hemodilution is less likely to occur.
In the crossmatch procedure, washed erythrocytes are incubated with serum. For horses, a source of complement (to hemolyse the erythrocytes) is added, as the antibodies in horses are usually hemolysins that require a source of complement for their hemolytic action.
For the major crossmatch: Donor erythrocytes are washed and incubated with recipient serum.
For the minor crossmatch: Donor serum is incubated with washed recipient erythrocytes.
The mixture of erythrocytes and serum are then observed visually for hemolysis (especially in the horse) and microscopically for agglutination. Any evidence of agglutination or hemolysis indicates an incompatible crossmatch.
When there is an incompatible reaction on the major crossmatch, the donor blood should not be transfused under any circumstances. When there is an incompatible reaction on the minor crossmatch, the transfusion can go ahead. However, if the donated serum is likely to contribute substantially to the plasma volume of the recipient, the serum should be removed from the donor whole blood. The packed cells (washed or unwashed) should be reconstituted in sterile isotonic saline before infusion.
A specific type of crossmatch procedure is the mare-foal incompatibility test.
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