In cats, a single mismatched blood transfusion can be fatal within minutes due to naturally occurring alloantibodies. Unlike dogs, who can typically receive one unmatched transfusion without immediate hemolysis, cats must always be blood-typed before their first transfusion. The Blood Volume Estimates calculator helps determine the precise volume needed for each patient.
Dogs have over 12 recognized blood group systems, designated Dog Erythrocyte Antigen (DEA). The clinically most significant is DEA 1.1. Approximately 40-45% of dogs are DEA 1.1 positive. DEA 1.1 negative dogs are preferred as universal donors because they lack the most immunogenic antigen.
Dogs do not typically have naturally occurring alloantibodies, which means a first transfusion from a DEA 1.1 positive donor to a DEA 1.1 negative recipient usually does not cause an immediate hemolytic reaction. However, the recipient will become sensitized, and a second mismatched transfusion can cause a severe, potentially fatal acute hemolytic reaction within minutes to hours. For this reason, typing and crossmatching are strongly recommended even for first-time transfusions.
Other DEA types (DEA 3, 4, 5, 7) are less clinically significant but can contribute to delayed transfusion reactions. DEA 4 is present in over 98% of dogs and is considered essentially universal.
Cats have three blood types: A, B, and AB. Type A is most common (approximately 95% in domestic shorthairs in the United States). Type B prevalence varies dramatically by breed: British Shorthairs (40-60% type B), Devon Rex (40%), Cornish Rex (30%), and Abyssinians (20%). Type AB is rare (<1% in most populations).
Critically, cats have strong naturally occurring alloantibodies. Type B cats have potent anti-A antibodies that cause rapid intravascular hemolysis when exposed to type A blood. Type A cats have weaker anti-B antibodies that cause a milder but still significant reaction. Type AB cats lack alloantibodies and can theoretically receive from either type, but type A blood is preferred.
Blood typing identifies the patient's blood group antigens. In-clinic card-based typing kits (e.g., RapidVet-H) use reagent-coated cards that agglutinate in the presence of specific antigens. Results are available in 2-5 minutes. Gel column typing (e.g., DiaMed) provides more sensitive results and is used in reference laboratories.
Crossmatching detects antibodies in the recipient's serum against donor red blood cells (major crossmatch) and antibodies in the donor's serum against recipient red blood cells (minor crossmatch). A major crossmatch is essential for any patient that has received a previous transfusion or where transfusion history is unknown. The test involves mixing recipient serum with donor cells (and vice versa) and observing for agglutination or hemolysis after incubation.
Crossmatching should be performed for all cats (given naturally occurring antibodies), any dog that has been previously transfused, any patient with suspected immune-mediated hemolytic anemia, and whenever the transfusion history is unknown.
The decision to transfuse is based on clinical assessment, not a single PCV cutoff. General guidelines:
- PCV <20% with clinical signs of anemia (tachycardia, weakness, tachypnea)
- Acute hemorrhage with >30% blood volume loss
- PCV <10-12% regardless of clinical signs
- Coagulopathy with active bleeding (fresh frozen plasma)
Packed red blood cells (pRBCs): Concentrated RBCs with most plasma removed. Used for anemia when volume expansion is not needed. Typical dose: 10-15 mL/kg in dogs; 5-10 mL/kg in cats.
Fresh frozen plasma (FFP): Contains all clotting factors, albumin, and immunoglobulins. Used for coagulopathies (rodenticide toxicity, DIC, liver failure). Dose: 10-20 mL/kg.
Fresh whole blood: Contains RBCs, platelets, clotting factors, and plasma. Indicated for massive hemorrhage requiring volume, oxygen-carrying capacity, and hemostasis simultaneously. Must be used within 8 hours of collection.
All blood products must be administered through a blood administration set with a 170-micron in-line filter to remove clots and aggregates. Never add medications to blood products. Use a dedicated IV line.
| Parameter | Initial Rate (First 15-30 min) | Maintenance Rate | Maximum Duration |
|---|---|---|---|
| Dogs (pRBCs) | 0.5-1 mL/kg/hr | 5-10 mL/kg/hr | 4 hours |
| Cats (pRBCs) | 0.25-0.5 mL/kg/hr | 3-5 mL/kg/hr | 4 hours |
| FFP (dogs/cats) | 1-2 mL/kg/hr | 5-10 mL/kg/hr | 4 hours |
| Cardiac patients | 0.25 mL/kg/hr | 1-3 mL/kg/hr | 4 hours (slower) |
Monitor vital signs (temperature, heart rate, respiratory rate, blood pressure) before transfusion, every 15 minutes for the first hour, then every 30 minutes until completion. Watch for facial swelling, urticaria, vomiting, fever, hemoglobinuria, and restlessness.
Acute hemolytic reaction: The most severe reaction, caused by antibody-mediated destruction of donor RBCs. Signs include fever, hemoglobinemia, hemoglobinuria, tachycardia, hypotension, vomiting, and collapse. Onset is within minutes to hours. Stop the transfusion immediately, maintain IV access with crystalloids, and treat for shock. This is life-threatening.
Febrile non-hemolytic reaction: Temperature increase ≥1°C during or within 4 hours of transfusion without hemolysis. Caused by recipient antibodies against donor leukocyte antigens or cytokines in stored products. Slow or pause the transfusion and administer diphenhydramine (1-2 mg/kg IM or slow IV). May resume at slower rate if fever resolves.
Allergic reaction (urticaria/anaphylaxis): Type I hypersensitivity to donor plasma proteins. Mild reactions present as facial swelling and hives; severe reactions progress to bronchospasm, hypotension, and anaphylaxis. Treat with diphenhydramine for mild reactions; epinephrine (0.01-0.02 mg/kg IV or IM) for anaphylaxis.
Volume overload: Particularly a risk in cats and cardiac patients. Signs include tachypnea, dyspnea, cough, and pulmonary crackles. Slow or stop the transfusion and administer furosemide (1-2 mg/kg IV).
Warning: Never assume a transfusion reaction is "just allergic." Acute hemolytic reactions can initially present with mild signs that rapidly progress to cardiovascular collapse. At the first sign of any adverse reaction, stop the transfusion, disconnect the blood product (keep the IV line), and reassess the patient completely before making a treatment decision.
Use the Blood Volume Estimates tool to calculate required transfusion volumes, and consult the Triage/Emergency Specialist for guidance on managing transfusion reactions. The Vital Signs Reference provides species-specific monitoring parameters.
- Always type cats before transfusion — naturally occurring alloantibodies make mismatched transfusions immediately life-threatening.
- Crossmatch all previously transfused patients — sensitization from prior transfusions causes severe delayed reactions.
- Start slow, monitor frequently — 0.5-1 mL/kg/hr for the first 15-30 minutes; vitals every 15 minutes for the first hour.
- Use a 170-micron filter — all blood products require a blood administration set; complete transfusion within 4 hours.
- Stop immediately for any reaction — maintain IV access, reassess completely, and treat before considering resumption.