When blood is needed in an emergency, there is no time to hesitate. Yet, every decision matters. Transfusion medicine in veterinary emergency care has advanced rapidly, but it remains one of the most judgment-dependent interventions we make. Unlike fluid resuscitation or oxygen supplementation, blood additionally carries immunologic and non-immunologic risks, comes with supply limitations, and often raises questions that do not have straightforward answers. When is a transfusion truly necessary? What product is most appropriate? How do we balance urgency with safety when time, diagnostics, and blood products are limited? Whether you are facing a crashing immune-mediated hemolytic anemia (IMHA) case, a trauma patient with active bleeding, or a cat with anemia of an undiagnosed cause, the goal is the same: deliver the right product, at the right time, in the safest possible way. Knowing when to transfuse Transfusion decisions are rarely black and white. While a low packed cell volume (PCV) may seem like a clear trigger, a transfusion should be guided by clinical context, not a number alone. The goal of a red blood cell (RBC) transfusion is to restore oxygen delivery, and not simply to normalize lab values. Oxygen delivery vs. oxygen content The body’s ability to deliver oxygen depends on more than just hemoglobin concentration. Cardiac output, oxygen saturation, tissue demand, and compensatory mechanisms (like increased oxygen extraction) all influence whether the patient truly needs a transfusion. Some patients with PCVs in the low teens may be stable and compensated, while others with higher PCVs may show signs of collapse due to concurrent disease or the acuteness of the decline. The critical oxygen delivery threshold (DO2crit) is the point at which oxygen consumption (VO2) becomes limited by supply rather than demand. This is when the body is no longer able to extract enough oxygen from the blood to meet tissue demands. When this happens, even maximal extraction can’t compensate, and tissues begin to experience hypoxia. Unfortunately, this threshold varies and is hard to measure directly in clinical practice. That is why transfusion decisions are made based on clinical signs of hypoxia (See: “Clinical indicators that support a transfusion”) instead of lab values alone. Clinical indicators that support a transfusion Physical parameters to monitor: Mucous membranes: ○ Pale, white, or gray mucous membranes suggest inadequate tissue perfusion. Heart rate: ○ Sustained tachycardia (>160 bpm in dogs, >200 bpm in cats) reflecting compensation. ○ Severe tachycardia (>200 bpm in dogs, >240 bpm in cats) or bradycardia (<60 bpm in dogs, <120 bpm in cats) may indicate decompensation or imminent arrest. Respiratory rate and effort: ○ Tachypnea at rest (>40 bpm) without primary respiratory disease. ○ Gasping, open-mouth breathing, or labored effort suggests an oxygen crisis. Mentation: ○ Dull, lethargic, or decreased responsiveness. ○ Stuporous or comatose mentation signals severe compromise. Activity tolerance: ○ Noticeable weakness, reluctance to move, or collapse. ○ Inability to stand or rise is a red flag. ● Laboratory values Laboratory values play a key supporting role in transfusion decision-making by helping assess the severity and trajectory of anemia, especially when paired with clinical signs. The most used marker is the PCV, which provides a quick estimate of red blood cell mass. However, PCV must always be interpreted alongside the patient’s clinical status. A single low PCV in a well-compensated patient may not require transfusion, while a dropping PCV in the context of weakness, tachycardia, or hypotension may support a more urgent decision. Serial PCV trends are often more valuable than a single value, particularly in cases of hemorrhage or hemolysis. Total solids (TS) add additional context. A low PCV with low TS suggests acute blood loss, while a low PCV with normal or elevated TS may point to hemolysis or chronic anemia. When both PCV and TS are trending downward, especially in a symptomatic patient, transfusion becomes more strongly indicated. Another important value is blood lactate, which serves as an indirect marker of tissue oxygen debt. Elevated lactate (>3 mmol/L) that does not improve with fluid resuscitation may indicate inadequate oxygen delivery due to anemia and supports consideration for transfusion even if PCV alone is not critical. In some cases, lactate may help identify patients who are compensating poorly despite what appears to be a borderline PCV. Choosing the right blood product Once the decision to transfuse has been made, the next critical step is selecting the appropriate blood product. Each product serves a different purpose and using the wrong one can lead to unnecessary risks, ineffective treatment, or wasted resources. Whole blood, packed red blood cells (PRBCs), plasma, and platelets each have specific indications and should be matched to the patient’s underlying need. Whole blood contains red blood cells, plasma proteins, clotting factors, and functional platelets if used fresh. It is most appropriate for patients experiencing acute hemorrhage with concurrent volume loss, especially in situations where time or inventory limits prevent component therapy. However, its volume load can pose risks in smaller or cardiac-compromised patients, and its shelf life is short (~28 days). PRBCs, on the other hand, are ideal for patients that need oxygen-carrying capacity support but are otherwise normovolemic. They allow for intentional RBC dosing without excessive volume. PRBCs are the product of choice in cases of anemia without active bleeding or coagulopathy, allowing clinicians to correct red cell mass without administering unnecessary plasma, also reducing the risks of immunologic complications (Figure 1). Figure 1. Veterinary hospitals can bank blood components, such as PRBCs, by investing in blood storage devices that provide reliable temperature control. Photo courtesy Kenichiro Yagi Plasma is appropriate when there is active bleeding due to documented coagulopathy, such as prolonged PT or aPTT, or in specific conditions, like rodenticide toxicity. It is not a volume expander, nor is it indicated in stable anemic patients. Similarly, platelet products, including fresh whole blood, platelet-rich plasma, and platelet concentrates, are only indicated in cases of severe thrombocytopenia with active bleeding, or for patients with inherited platelet function disorders. In immune-mediated thrombocytopenia, transfused platelets are often rapidly destroyed, limiting their utility to critical cases such as intracranial or pulmonary hemorrhage. Ultimately, the goal is to match the product to the patient’s specific deficit. Anemic patients need red cells. Coagulopathic patients with bleeding from coagulation factor deficits need clotting factors. Thrombocytopenic patients with active hemorrhage need platelets. Transfusing the right component improves outcomes, minimizes risks, and respects the fact that blood products are not generic support tools. They are targeted, lifesaving interventions that should be used with intention. Transfusion compatibility: Not always ‘free’ The idea that dogs can safely receive a first unmatched transfusion has been a longstanding belief in veterinary medicine, but it deserves careful reconsideration. While it is true most dogs lack naturally occurring alloantibodies against other dog erythrocyte antigens (DEAs), calling the first transfusion “free” oversimplifies a complex process. Sensitization can occur even after a single mismatched transfusion, and delayed hemolytic reactions, although not immediate, can still cause significant clinical deterioration days later. For example, dogs mismatched for DEA 1 become sensitized after their first transfusion, developing alloantibodies within several days. These dogs are then at risk for acute hemolytic reactions upon any subsequent exposure. Even without prior transfusions, some dogs may have low-level sensitization from pregnancy, transfused plasma, or unknown prior exposure. The notion that the first transfusion is “risk-free” encourages complacency and overlooks the very real potential for harm. The safest blood to give without crossmatching is from dogs that have tested negative for all major alloantigens, particularly DEA 1. The emerging importance of less commonly tested antigens, such as Dal, is gaining attention, particularly in certain breeds like Dalmatians, Dobermans, and Shih Tzus, which may lack the Dal antigen. When exposed to Dal-positive blood, dogs negative for Dal can develop alloantibodies and be at risk for future transfusion reactions. Unfortunately, Dal and other minor antigens are not routinely typed in clinical practice (though a blood typing kit is now available), increasing the chance of sensitization even with DEA type-matched blood. In cats, the stakes are even higher. Unlike dogs, cats have naturally occurring alloantibodies against incompatible blood types, making the first transfusion potentially fatal if type mismatched. A type B cat given type A blood, even once, can suffer an acute, often fatal reaction involving hemolysis within minutes. For this reason, blood typing and cross-matching before any feline transfusion, regardless of history, is recommended. In addition to the familiar AB blood group system, other blood group antigens, such as Mik, have been identified in cats and are thought to play a role in transfusion incompatibility even when the major type is matched. Emergency readiness: When whole blood is the best option In practices that do not routinely store blood components, being prepared to call a donor in to collect and administer fresh whole blood on short notice is critical. In many emergencies, such as trauma, surgical hemorrhage, or anemia with concurrent hypovolemia, fresh whole blood may be the most practical and effective option (Figure 2). It delivers red blood cells, clotting factors, plasma proteins, and platelets in one product, with no time lost to sourcing external units. Figure 2. A dog receives a whole blood transfusion by gravity drip. Photo courtesy Kenichiro Yagi Clinics should have a protocol for identifying and screening emergency donors. Dog donors are ideally healthy, vaccinated, weigh more than 25 kg, temperamentally suited for donation, and negative for in-house infectious disease screening tests (e.g. SNAP tests for heartworm antigen, Anaplasma, and Ehrlichia). In a true emergency, minimal testing, such as PCV/TS, DEA 1 typing, and a brief health history, can be acceptable to assess donor suitability. If DEA 1 negative blood is not available, documenting the recipient’s blood type and avoiding repeat transfusions of DEA 1 positive blood in the same patient becomes essential. For cats, ideal candidates are healthy, indoor-only, at least 4.5 kg, up to date on vaccines, free of clinical illness, and negative for FeLV and FIV (tested within the past year or at the time of donation). In a true emergency screening such as PCV/TS, a brief physical exam, and rapid blood typing (for both donor and recipient) is the minimum standard. If crossmatching is not feasible, whole blood from a known-type, compatible (A to A, B to B) donor should be used with extreme care. Fresh whole blood should be used within six to eight hours to preserve platelet and coagulation factor activity and collected into appropriate anticoagulant bags (e.g. CPDA-1). Having these supplies on hand, along with a clearly designated space and team members trained in collection (Figure 3), helps ensure that a source of whole blood is available. Figure 3. A canine blood donation. Whole blood contains red blood cells, plasma proteins, clotting factors, and functional platelets if used fresh. Photo courtesy Kenichiro Yagi Emergency options and last resorts In the most urgent situations, when a patient needs blood and none is available, veterinary teams may be forced to consider options outside the norm. These “last resort” strategies are used rarely and with caution, but in some emergency scenarios, they may be the only path to survival. Two such options include xenotransfusions and autotransfusion. Xenotransfusions: Dog to cat transfusions A xenotransfusion is the transfusion of blood from one species to another, most commonly, dog to cat. This is typically performed only when feline blood is unavailable, and the patient is in imminent danger of death. Dogs are often used as donors because their blood is easier to obtain in sufficient volume, and cats generally lack naturally occurring alloantibodies against canine red blood cells. If a cat survives a xenotransfusion, the transfused dog red cells will typically be destroyed within four to seven days. Additionally, there is a significant risk of severe hemolytic reactions, sensitization, and immune-mediated complications if a second xenotransfusion is attempted. Because of these risks, xenotransfusions should be considered only once, as a bridge to stabilization until compatible feline blood can be sourced. They are best reserved for life-threatening anemia, when the alternative is death, and should be discussed clearly with the client as an ethical exception and last resort, not a standard of care. Autotransfusion: Patient as their own donor Autotransfusion involves collecting and reinfusing a patient’s own blood, typically from the pleural or peritoneal space after trauma or internal hemorrhage. This technique can be lifesaving, especially in cases such as hemoabdomen or hemothorax (Figure 4A), where large volumes of blood have accumulated and no donor blood is available. In some cases, their own blood may be the only source of compatible blood in animals of rare species or blood type. Figure 4A. Blood being evacuated via thoracocentesis in a dog with hemothorax. Photo courtesy Kenichiro Yagi Autotransfusion eliminates the risk of incompatibility, and in many cases, the blood has only recently left the vascular system and remains viable. Blood can be collected via sterile suction and then transferred into an IV bag and be spiked with a filter administration set for transfusion. Other methods include the use of a cell saver device (if available), although basic techniques using syringes with a three-way stopcock or collected into syringes (Figure 4B) and syringe filters are often sufficient. Figure 4B. Syringes filled with blood from the thorax are stored aseptically and are ready to be transfused if warranted. Photo courtesy Kenichiro Yagi There are, however, contraindications. Blood contaminated with urine, bile, bacteria, or tumor cells should not be reinfused. Intraoperative or traumatic sites should be carefully evaluated, and the decision to autotransfuse must balance the urgency of the situation against the potential for introducing harmful substances. When used selectively and with proper technique, autotransfusion can serve as a valuable, low-risk option in critical care settings, particularly in large-breed dogs where volume loss is significant. In true emergencies, these options can buy time and save lives, but they demand clinical judgment, informed consent, and a clear plan for what comes next. Whether using another species or the patient’s own blood, these measures are not substitutes for standard transfusion practice, but in the right moment, they can mean the difference between survival and loss. Transfuse with purpose Emergency transfusions demand timely action, clear reasoning, and a deep understanding of each patient’s clinical picture. Decisions should be based on physical signs and trajectory instead of relying on lab values, with a focus on matching the right product to the specific deficit. As our understanding of transfusion medicine evolves, so does our practice. This means replacing outdated assumptions with current evidence and using blood products intentionally. Veterinary professionals should recognize when transfusions are needed, ensure they are delivered safely, and monitor for complications. Doing so improves patient outcomes and reinforces a culture of precision, responsibility, and teamwork in one of the most critical aspects of emergency care. Kenichiro Yagi, MS, RVT, VTS (ECC, SAIM), is chief veterinary nursing officer at VEG ER for Pets and program director for RECOVER. A recognized leader in ECC and transfusion medicine, Yagi publishes, speaks globally, and advocates for veterinary nursing, inspiring others to challenge norms and push the profession to new heights.
