To say that transfusion medicine has an increasingly important role in the life support of critically ill patients is an under statement. The knowledge that veterinary professionals have on companion animal transfusion therapy has drastically intensified since the first blood typing experiments were done on canines back in the 1960s. Today, the veterinary community has not only recognized the many different blood groups for canines, but our understanding of what exactly happens to an animal when a transfusion is administered has revolutionized how we can be certain we are providing the right therapy for our patients.
It may sound trivial, but transfusion medicine all starts with a blood type. Blood types are genetic markers on RBC surfaces that are antigenic and species specific. In other words, the presence of a specific antigen on a red blood cell surface is unique and determines that animal’s blood type. Furthermore, a set of blood types of two or more alleles comprises a blood group system. Think of alleles as variations of a gene—one allele can give a canine a positive type while another can give it a negative type. There are over a dozen blood group systems that have been recognized in dogs; however, only six are clinically significant. These are referred to as dog erythrocyte antigens, with the abbreviation DEA followed by a number. So far, the recognized blood types in dogs are DEA 1:1, 1:2, 3, 4, 5, and 7.
So what does this all mean? When you blood type a canine with an in–house kit, you are determining whether the dog is testing positive for DEA 1:1 (giving it a positive blood type) or negative for DEA 1:1 (giving it a negative blood type). Furthermore, complete blood typing (which can only be performed at a specialized lab) can show if that animal is also positive or negative for the remaining antigens 3–7. For example, a dog that is blood typed DEA 1:1 negative can also be positive for the DEA 3 antigen and so forth. On the other hand, a dog can be typed DEA 1:1 positive and test negative for the remaining antigens. The only recognized “universal” type is the dog that is negative for all of the anti¬gens except DEA 4; laying to rest the claim that all negative dogs could be “universal donors.”
So why is this important? Why can’t I just give a dog negative or positive blood? Canine serum can contain antibodies against a specific antigen, especially if that animal has received a transfusion in the past. The DEA 1:1 and 1:2 are the most antigenic and are of the most clinical importance. Delivery of DEA 1:1 positive blood to a negative dog (especially a dog that has received a previous transfusion) will likely elicit a strong alloantibody response otherwise known as an acute immune mediated transfusion reaction. Clinical signs of such a reaction are associated with an anaphylac¬tic reaction and may include tachycardia, vomiting, shock, hemolysis and possibly cardiac arrest. Signs can occur as early as five minutes into a transfusion with the administration of only a few milliliters of the wrong blood Reactions can also occur in canines that are negative for DEA 3, 5 and 7 and receive blood that is positive for those antigens. These reactions can be immediate or delayed, resulting in hypersensitivity anywhere from 3–14 days post–transfusion. Signs may include fever, falling PCV or hyperbilirubinemia.
The take home point is to ensure that the blood product you are about to deliver is compatible with your patient. The best way to do this is to cross–match your donor’s blood to your patient. Whereas blood–typing tests reveal the blood group antigens on the red cell surface, blood cross–matching tests indicate the serologic compatibility or incompatibility between donor and recipient. A major cross–match is performed by combining the donor’s red blood cells to the recipient’s serum. In essence, this is done to detect the presence of alloantibodies in your patient that would react to or reject a donor’s red blood cell antigens. A minor cross–match is performed by combining a recipient’s red blood cells to a donor’s serum, detecting an incompatibility of a donor’s antibodies against a recipient’s red blood cell antigens. A major cross–match is of greater concern because it can predict whether or not a patient will attack the red cells it receives, which can result in an acute hemolytic reaction (meaning the patient’s immune system will attack and destroy the transfused red cells). This means that reasons behind giving the transfusion, whether it is anemia, hemorrhage or decreased oxygen–carrying capacity are null and void because the patient will be receiving no benefit from it. Canines receiving a blood transfusion can develop antibodies against a specific antigen; therefore, they should absolutely be cross–matched for any subsequent blood transfusions.
If a dog lacks any antigens on its own red blood cells, receiving a transfusion containing “foreign” antigens will most likely elicit an immune response. An immune response can mean anything from a complete anaphylactic reaction that can result in death to delayed hemolysis of those red cells days to weeks later. Either way, you’re eliminating the reason for the transfusion in the first place.
Like most theories in this world, we have learned an extensive amount of knowledge through experimentation. The original idea that dogs simply had two different blood types and that we could easily transfuse them with blood from a donor of the same type is an idea of the past. The discovery of different antigens on the red blood cell surface and the adverse effects of antigen/antibody relationships of dogs have provided veterinary professionals with a more advanced outlook on transfusion therapy. Although it is not possible to immediately know all the antigens that a critically ill patient’s blood possesses, knowing their blood type for certain and knowing the specific antigens of our blood donors can greatly affect the outcome of any necessary transfusion. Through the use of blood typing and cross–matching, we can predict whether a patient receiving a blood transfusion will, first and foremost, be compatible with it and that the transfusion will deliver its desired effect. Knowing the contrary relationship of antigens and antibodies, we can safely assume that the introduction of any foreign antigen into a patient has the potential to stimulate the patient’s immune system to respond negatively. So remember, strive to obtain a canine’s blood type before offering any kind of transfusion. If a blood type is unavailable, transfusion with “universal” (that is blood that lacks any undesired antigens) blood should be the first choice. This will hopefully prevent any negative immune response and allow a transfusion to achieve its desired affect. Most importantly, cross–matching should always occur beforehand to ensure that the donor’s blood is compatible with the patient.