Red Blood Cell Transfusions

The most common indication for transfusions in dogs and cats is anemia (see Chapter 120, Anemia). Transfusions are generally required after loss of the blood’s oxygen-carrying capacity (i.e., loss of hemoglobin) and subsequent tissue/organ ischemia, and not as a simple volume expander. Depending on the type, degree, rapidity, and course of the anemia, a transfusion with blood products, such as stored packed red blood cells (pRBCs), fresh whole blood FWB, or stored whole blood, may be warranted. Animals with rapidly progressive anemia should be transfused when the Hct is approximately 20% to 25%, and a patient with chronic anemia may not require transfusion despite having a much lower Hct.

Healthy animals can readily tolerate a loss of up to 20% of blood volume (blood donors regularly give 20 ml/kg body weight q6-12wk) without any ill effects. However, animals with acute hemorrhage exceeding 20% of the blood volume may require a blood transfusion in addition to the initial shock fluid therapy (see Chapter 65, Shock Fluids and Fluid Challenge). It should be noted that animals with peracute blood loss will not show a drop in Hct for hours following hemorrhage, until intercompartmental fluid shifts occur or fluid therapy is instituted. Hence, other parameters are used to decide if transfusion therapy is indicated, such as mucous membrane color, capillary refill time, heart rate, blood pre-ssure, and possibly blood lactate levels. An assessment of an arterial blood gas and respiratory rate and effort are useful to evaluate animals with coexisting respiratory disease (ventilation-perfusion mismatch). In most animals with anemia secondary to acute blood loss, fluid therapy alone will restore vital organ perfusion, although pRBCs should be considered in any animal with evidence of tissue hypoxia. A falling Hct is not a contraindication of fluid administration, although excessive blood collection for diagnostic tests may necessitate blood replacement in a sick animal. Animals that require anesthesia and surgery should have a Hct of at least 20% to ensure adequate oxygen-carrying capacity during anesthesia.

In animals with immune-mediated hemolytic anemia (see Chapter 121, Acute Hemolytic Disorders), red blood cell transfusions have proven lifesaving and there is no evidence that transfused red blood cells are destroyed more rapidly than the patient’s own erythrocytes. There are no data to show that the transfused cells “add fuel to the fire.” The administration of a bovine hemoglobin solution (Oxyglobin) has also shown beneficial effects.

Fresh Frozen Plasma

Fresh frozen plasma (FFP) is used most commonly in veterinary practice to treat coagulopathies causing serious bleeding, because this product contains all coagulation factors. FFP is commonly used in animals with hemorrhage secondary to acquired coagulopathies (i.e., liver disease and anticoagulant rodenticide intoxications) or patients with hereditary coagulopathies and subsequent bleeding. Sudden hemorrhage caused by therapeutically used heparin (including accidental use of undiluted heparin flushes) or warfarin to counter thrombosis can also be corrected with FFP, although protamine can also reverse the heparin-induced effects. The use of FFP (with or without heparin) to replace deficient coagulation factors and antithrombin to treat patients with immune-mediated hemolytic anemia or disseminated intravascular coagulation is controversial. There are no studies documenting a definitive beneficial effect. Similarly, evidence for the use of FFP in animals with acute pancreatitis (to replace α-macroglobulins and antiproteases) or in parvovirosis (to provide antiparvovirus antibodies and additional immunoglobulins and to stop gastrointestinal hemorrhage) is lacking. FFP is also commonly used to correct hypoproteinemias in cases of protein-losing nephropathies and enteropathies, but its effect on oncotic pressure in these animals is minimal at clinically used dosages, especially when compared with synthetic hyperoncotic agents such as dextran-70 or hydroxyethyl starch. Critically ill animals with albumin concentrations less than 1.5 g/dl may benefit from plasma therapy, because this protein is an important carrier of certain drugs, hormones, metals, chemicals, toxins, and enzymes.

Other Blood Products

Other blood products are used less commonly in dogs and are not generally available for cats. Cryoprecipitate is rich in fibrinogen, fibronectin, factor VIII, and von Willebrand factor and is the preferred treatment for bleeding dogs with these plasma protein deficiencies. If available, cryoprecipitate-poor plasma may be administered to many coagulopathic and hypoproteinemic dogs when synthetic plasma expanders are of limited use or have undesirable side effects. Because platelets are relatively short-lived (1 week) and cannot readily be stored for any length of time (<24 hours at room temperature), they rarely are transfused. Hemorrhage caused by thrombocytopenia in anemic dogs could be treated with FWB, but generally requires only pRBCs to correct the anemia. Rarely, platelet-rich plasma (PRP) and platelet concentrates are required to control life-threatening bleeding (see Chapter 119, Thrombocytopenia). Furthermore, in dogs with immune-mediated thrombocytopenia, transfused platelets have a very short half-life (hours) and will not result in any appreciable platelet rise, but they may transiently stop severe hemorrhage. Althouh cryopreserved platelets are currently available for use in dogs, there is insufficient information to document their efficacy or to recommend routine use of the product. Research is ongoing. Because of the very short half-life of granulocytes (hours), leukocyte transfusions are not generally practiced in human or veterinary medicine.¹

Canine Blood Types

Dogs have more than a dozen blood group systems known as DEAs. Canine erythrocytes are either positive or negative for a blood type (e.g., DEA 4 positive or negative), and these blood types are thought to be codominantly inherited. In the DEA 1 system, which represents an exception, DEA 1.1 (A₁) and 1.2 (A₂) are allelic, and there may even be a DEA 1.3 (A₃). Thus a dog can be DEA 1.1 positive or DEA 1.1 negative, and only DEA 1.1-negative dogs can be DEA 1.2 positive or DEA 1.2 negative. There are no clinically important alloantibodies present before sensitization of a dog with a transfusion (pregnancy has never been reported to cause sensitization).^2,4,5

The most important canine blood type is DEA 1.1. DEA 1.1 (A₁) elicits a strong alloantibody response after sensitization of a DEA 1.1-negative dog by a DEA 1.1-positive transfusion. This can lead to an acute hemolytic transfusion reaction in a DEA 1.1-negative dog previously transfused with DEA 1.1-positive blood. Transfusion reactions against other blood types in previously transfused dogs have been described rarely. They include reactions against the DEA 4,⁶ Dal in Dalmatians, and another common red blood cell antigen in a Whippet. Additional clinically important blood types may yet be discovered.⁷

Simple blood typing cards using a DEA 1.1 monoclonal antibody are available for DEA 1.1 typing of dogs (DMS Laboratories, Flemington, NJ), but there is some concern that weak agglutination reactions may be caused by the DEA 1.2 antigen. More recently, a reliable gel column procedure has been introduced (DiaMed, Cressier, Switzerland) for clinical pathology laboratories. Ethylenediaminetetraacetic acid is preferred over other anticoagulants.⁸ Dogs that are DEA 1.1 negative are considered universal blood donors for a dog that has never been transfused. Canine blood typing sera for DEA 1.1, 1.2, 3, 4, and 7 and limited typing services are available (Midwest Blood Services, East Lansing, MI), but other blood groups of clinical importance can be identified by crossmatching previously transfused dogs. Persistent autoagglutination following saline washing of the recipient’s blood negates any typing and crossmatch testing. Unless a bitch has been transfused previously, there is no concern for NI.