The feline blood group system is defined as the AB system, with cats being type A, B, or AB. Type A is the most common blood type in cats, and type B prevalence varies by breed and geographic location. Cats are born with naturally occurring alloantibodies against blood types other than their own. Severe acute hemolytic reactions can occur if a type B cat receives type A blood; type B blood given to a type A cat has shortened red cell survival time. Discovery of the feline red cell antigen, Mik, raised further questions about red cell antigens outside the classic AB system and their clinical relevance.

This study aimed to estimate the prevalence of cats with non-AB red cell antigens and to begin identifying the number of potential non-AB red cell antigens. Blood samples were collected from 11 blood donor colony cats, 24 research colony cats, and 102 client-owned healthy or sick cats; 134 surplus EDTA blood samples were also collected from healthy or sick cats. AB blood typing was performed, and 13 type B and AB cats were excluded. The remaining 258 type-A cats were subsequently evaluated for the presence of non-AB alloantibodies. Extensive crossmatching revealed 18 cats with unidentified non-AB alloantibodies. Of these, only 7 cats were available to have new blood samples drawn for more extensive crossmatching, which identified 5 potential feline erythrocyte antigens outside the AB classification system.

Key pearls to put into practice:

Cats have naturally occurring alloantibodies against the blood type they do not express. Based on the AB blood type system, administration of type A blood to a type B cat can cause an acute hemolytic transfusion reaction.

There may be other unidentified non-AB red cell antigens that could lead to transfusion incompatibility. This study identified the presence of 5 potential red cell antigens outside the AB classification system in a low number (7/258) of cats.

Blood typing prior to any transfusion in cats is recommended. Crossmatching prior to transfusion can be considered to identify non-AB incompatibilities. Cats that have previously received a blood transfusion should be crossmatched prior to transfusion.