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Hematology & Transfusion · Clinical background

Transfusion

Volume, rate, and reaction management for pRBC, whole blood, and FFP transfusions in dogs and cats.

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Transfusion — clinical background

Transfusion is one of the most decision-dense interventions in small-animal critical care. Choosing the wrong product, transfusing too fast, or missing an early reaction can turn a salvage measure into the proximate cause of decompensation. This article covers product selection, volume calculation, rate, monitoring, and reactions, with the reasoning behind the numbers in the calculator.

When to transfuse

There is no single PCV threshold that triggers transfusion. The decision is clinical, anchored on whether the patient is symptomatic for anemia, the rate at which the PCV is changing, and the underlying cause.

For chronic, slowly progressive anemia (chronic kidney disease, occult bleeding from a mass), patients often tolerate a PCV of 12-15% if they are not actively decompensating. Acute anemia of the same severity (immune-mediated hemolytic anemia, acute trauma) typically requires transfusion because there has been no time for compensatory adaptation. Signs of decompensation that should drive the decision: weakness, tachycardia, tachypnea, lactate above 3 mmol/L, hypotension despite fluid resuscitation, mentation changes.

In an actively bleeding patient, transfuse early. Do not wait for the PCV to fall below a threshold. Whole blood or pRBC plus FFP is the right combination for hemorrhagic shock.

Product selection

Three products cover most of small-animal practice.

Packed red blood cells (pRBC) are the first choice for restoring oxygen-carrying capacity in a non-bleeding patient with anemia (IMHA, chronic anemia, blood loss already controlled). PCV of a pRBC unit is approximately 80%. Volume is calculated to raise PCV to a target value, typically 25-30%.

Whole blood carries red cells (PCV approximately 40-60%), plasma factors, and platelets. It is indicated when the patient needs both oxygen-carrying capacity and clotting factors (acute hemorrhage with coagulopathy, severe DIC). Fresh whole blood is also the only practical source of functional platelets in most clinics. Whole blood is preferred over component therapy in actively bleeding patients because the entire missing volume is restored at once.

Fresh frozen plasma (FFP) supplies clotting factors without red cells. It is used for vitamin K antagonist toxicity (anticoagulant rodenticide) that is bleeding before vitamin K has had time to take effect, hereditary factor deficiencies, and as part of multimodal resuscitation in DIC. FFP is not a volume expander; if hypovolemia is the problem, give crystalloids.

Cryoprecipitate and platelet-rich plasma exist but are rarely available outside referral centers.

Volume calculation

For pRBC and whole blood, the volume to transfuse is the volume of donor product that delivers the missing red cell mass:

V = (PCV_target − PCV_current) / PCV_donor × BV_recipient

where recipient blood volume is 90 mL/kg in dogs and 60 mL/kg in cats. Donor PCV is 80% by default for pRBC and 60% for whole blood. The calculator on the previous tab does this arithmetic with the patient’s weight, current PCV, and target PCV.

For FFP, dosing is weight-based: 10-20 mL/kg. Use 10 mL/kg for general factor replacement and 15-20 mL/kg for active bleeding or DIC. Doses above 20 mL/kg are unusual outside referral medicine and risk volume overload, especially in cats.

A practical rule for pRBC in dogs: one full unit (~250 mL) raises PCV by roughly 5-10% in a 25-kg dog. Cats are smaller, so a single cat-sized unit (50-60 mL) achieves a similar rise.

Rate

Two-rate protocol. The first 30 minutes are a slow trial: deliver 25-50% of the calculated rate while watching for acute reactions (vomiting, urticaria, fever, tachycardia, dyspnea, weakness, hemoglobinuria). If the patient tolerates the slow trial, advance to the main rate for the remainder.

Total time for transfusion should not exceed 4 hours. After 4 hours at room temperature, bacterial contamination risk rises sharply, and the product should be discarded.

Patients at risk for volume overload (cats, patients with cardiac disease, very small dogs) should receive the transfusion over the full 4 hours and be monitored closely for tachypnea and crackles. If TACO develops, slow or stop the transfusion, give furosemide, and supplement oxygen.

Monitoring

Baseline vitals immediately before transfusion: temperature, pulse, respiratory rate, mucous membrane color, PCV/TS. During transfusion:

Recheck PCV at 1 hour post-transfusion to confirm the expected rise. If the post-transfusion PCV is significantly lower than expected, consider acute hemolytic reaction or ongoing blood loss.

Reactions

Reactions are categorized by mechanism and timing. Stop the transfusion immediately for any reaction; many are reversible if caught early.

Acute hemolytic reaction (immediate, life-threatening). Fever, vomiting, collapse, hemoglobinemia, hemoglobinuria, DIC. Most often from blood-type incompatibility (DEA 1.1 mismatch in dogs, AB system in cats). Stop the transfusion. IV crystalloids and supportive care. Diphenhydramine and steroids do not treat acute hemolysis; they are for allergic reactions.

Allergic or urticarial reaction (mild, common). Facial swelling, urticaria, pruritus, mild fever. Pause the transfusion, give diphenhydramine 1-2 mg/kg IM, resume at a slower rate once signs resolve. Pretreatment with diphenhydramine reduces recurrence in patients with a known reaction history.

Volume overload (TACO). Tachypnea, dyspnea, pulmonary crackles. More common in cats and patients with cardiac disease. Slow or stop the transfusion, give furosemide (1-2 mg/kg IV in dogs, 0.5-1 mg/kg IV in cats), supplement oxygen.

Delayed hemolytic reaction (3-14 days post). Falling PCV without overt signs of acute hemolysis. Confirms an anti-RBC antibody response. Future transfusions for this patient require crossmatching.

Febrile non-hemolytic reaction. Fever (>1.0°C rise) without other signs. Pause transfusion, rule out hemolysis, treat with antipyretics if needed, resume at slower rate.

Crossmatching and typing

In dogs, the most clinically important blood group is DEA 1. DEA 1-positive blood transfused into a DEA 1-negative recipient causes sensitization on first exposure and a hemolytic reaction on second exposure. First transfusions in DEA 1-negative dogs without prior typing are generally safe; subsequent transfusions in any dog should be crossmatched.

In cats, blood types A, B, and AB are clinically important and naturally occurring antibodies are present without prior exposure. Every cat transfusion should be typed and crossmatched before the first unit. Type B cats receiving type A blood develop fatal hemolysis within minutes.

Most modern in-house typing cards work for both species and take about 5 minutes. Major crossmatching adds another 15-30 minutes but is the only way to detect minor antigen incompatibility.

Discontinuation

Transfusion is complete when the calculated volume has been delivered or when PCV reaches target on intra-transfusion recheck. The slow trial period at the start cannot be skipped, even if the patient is in extremis. Most reactions occur in the first 30 minutes, and a missed acute hemolytic reaction will compound decompensation faster than continued anemia.

Post-transfusion, recheck PCV at 1 hour, 12 hours, and 24 hours. Monitor for delayed reactions for at least 14 days.

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