If you work in small animal practice, you are bound to one day find yourself somehow involved in giving a blood transfusion.

While the administration of blood transfusions can vary significantly, depending on whether you’re working in a general practice that doesn’t routinely perform transfusions or a high-volume emergency clinic, there are certain steps that should always remain constant. It’s important to have a grasp of these basic steps in pre-transfusion testing. 

The Backstory: Blood Types in Dogs and Cats 

In cats, blood types are very important. There are three feline blood types (Type A, Type B, and Type AB) and all of these cats except cats with Type AB blood are born with naturally-occurring antibodies against the other blood types. A lack of antibodies makes cats with Type AB blood a universal recipient, but there is no universal donor in cats. Therefore, it’s important to blood type cats prior to blood transfusion. 

Type A is the most common blood type in cats, including 95% of domestic shorthairs and longhairs, as well as the majority of Siamese, Burmese, Tonkinese, Russian Blue, American Shorthair, and Oriental Shorthair cats.¹ Type B is primarily found in Abyssinians, Birmans, British Shorthairs, Cornish Rexes, Devon Rexes, Maine Coons, Norwegian Forest Cats, Persians, Somalis, Sphinxes, and Scottish Folds, although even in these breeds it is often a minority of cats that have Type B blood.¹ Type AB blood is rare, but has been found in a variety of breeds. 

Summary of Major Feline Blood Types

In dogs, blood types play a less significant role in transfusion medicine. Although there are over a dozen different blood types in dogs,² it’s only dog erythrocyte antigen (DEA) 1.1 that plays a significant role in transfusion reactions. More importantly, no dog is born with naturally-occurring antibodies against DEA 1.1. (Okay, I know we should never say never, but this really is not a practical concern.) Dogs of other blood types only develop antibodies against DEA 1.1 after receiving a transfusion from a DEA 1.1 donor. This means that blood typing is not necessary for a dog’s first blood transfusion. It may still be recommended, to help ensure a longer half-life of the donor red blood cells, but it isn’t strictly necessary. It’s only when a dog is receiving repeated blood transfusions that we begin to worry about significant transfusion reactions.

Crossmatching 

While knowing the blood type of the donor and the recipient provides some assurance that a transfusion reaction is unlikely, there is an additional test that should be performed prior to most blood transfusions: a crossmatch. Crossmatching looks for serum antibodies against red blood cells. 

There are two types of crossmatch: major crossmatch and minor crossmatch. 

• Major crossmatch: tests for antibodies in the recipient serum against donor erythrocytes.
• Minor crossmatch: tests for antibodies in the donor plasma/serum against recipient erythrocytes.

Major crossmatching should always be performed prior to transfusion in cats. In dogs, a major crossmatch is only strictly required in dogs with a history of a transfusion, an unknown transfusion history, or a transfusion within the previous several days (even if the same donor will be used).¹ 

A minor crossmatch is typically considered less vital than a major crossmatch, but minor reactions can occur and therefore this test should also be considered in all transfusions.¹ 

Ideally, crossmatching should be performed using a commercially available test kit.² In an emergency setting, a slide crossmatch can be used as a crude method of assessment.¹ A slide crossmatch will fail to identify hemolytic reactions and other types of reactions, however, making it less beneficial than a commercially available crossmatch test.¹

Major Crossmatch: Slide Technique

1. Mix 2 drops of recipient plasma with 1 drop of donor blood on a slide. 
2. Rock the slide gently to mix the blood and plasma. 
3. Observe the slide macroscopically for evidence of agglutination. 
4. Observe the slide under a microscope for evidence of agglutination. 

Minor Crossmatch: Slide Technique

1. Mix 2 drops of donor plasma with 1 drop of recipient blood on a slide. 
2. Rock the slide gently to mix the blood and plasma. 
3. Observe the slide macroscopically for evidence of agglutination. 
4. Observe the slide under a microscope for evidence of agglutination. 

If agglutination is noted on a major crossmatch, DO NOT PROCEED WITH THE TRANSFUSION. Another donor should be selected.

If agglutination is noted on a minor crossmatch, proceed with caution. If possible, another donor should be selected. However, if necessary, the donor red blood cells can be washed to remove plasma.

Don’t Try to Remember Everything! 

If you’re in a practice that performs blood transfusions on a regular basis, all of this will likely become second-nature. In general practice, where blood transfusions are less common, it’s okay to use your resources to help you! Once you have a basic understanding of canine and feline blood types and crossmatching, you can easily use a textbook or other resource to help you with the step-by-step process of performing a blood transfusion. 

References

1. Vap LM. 2010. An update on blood typing, crossmatching, and doing no harm in transfusing dogs and cats. DVM360. Retrieved from: https://www.dvm360.com/view/update-blood-typing-crossmatching-and-doing-no-harm-transfusing-dogs-and-cats
2. Sirois, M. 2019. Blood Typing and Crossmatching. VetFolio. Retrieved from: https://www.vetfolio.com/learn/article/blood-typing-and-crossmatching