Blood transfusions in cats

C. Knottenbelt, M.W. Jackson and M. O’Brien

Blood or blood products are commonly required in surgical patients and a well-timed transfusion can be life-saving.1–3 The presurgical and pre-anesthetic workup should identify those patients that would benefit (see Chapter 1). Pre-existing anemia, anticipated extensive blood loss during surgery and known coagulation disorders are the most common indications for transfusing fresh whole blood or blood products. However, cats must NEVER receive any blood or blood products unless they have been blood typed, and preferably cross-matched, with the donor. The risk of fatal transfusion reactions is too great, even in first transfusions, if these steps are not taken.⁴ Feline blood groups are becoming increasingly complex and it is therefore impossible to safely ‘guess the blood type’. Typing is technically easy to perform and commercial kits are available allowing desk-top typing in the veterinary practice. For routine surgical patients, blood typing and cross-matching can be done at admission. However, for surgical emergencies blood typing could be done at induction or during surgery if blood or blood products are required. By following the guidelines below and using standard operating procedures, cats will receive the benefits of a safe blood product supply. This will enhance the quality of life for feline surgical patients requiring transfusion while minimizing the risks to both donor and recipient cats.

Assessing the need for a transfusion

Transfusion should be considered in any feline patient with a packed cell volume (PCV) of less than 20% that is due to undergo surgery or where it is indicated by the clinical condition of the patient.⁵ Reasons for considering an emergency transfusion are shown in Box 5-1. Although many surgical cases would benefit from blood component therapy (e.g., platelet rich plasma, fresh plasma for clotting factors),^6,7 this is rarely available for feline patients. In general, patients with hypovolemic anemia or those expected to have substantial blood loss during surgery should receive fresh whole blood. Patients with coagulopathies, especially secondary to hepatic disorders, and/or those that are normovolemic can be given component therapy where possible.⁶ Always collect any blood required for tests, such as typing, FeLV/FIV, Coomb test etc, BEFORE giving the patient a transfusion.

Box 5-1 Emergency transfusion is required in surgical patients when

The patient is exhibiting significant clinical signs of anemia (weakness, tachycardia, tachypnea, ataxia)

The packed cell volume is less than 10% or has fallen rapidly to less than 15%

Unexpected blood loss during surgery

Whole blood, colloids and crystalloids are used to restore blood volume. Transfusion is only required if bleeding is severe enough to result in anemia. Healthy conscious cats can lose up to 20% of their blood volume (13 mL/kg) without adverse effects provided that circulation is supported with colloids or crystalloids. In an anesthetized patient, blood loss of greater than 10% (equivalent to 7 mL/kg) should be considered potentially life-threatening.⁸ Assessment of peripheral oxygenation is an insensitive way of assessing if blood loss is significant, as most anemic patients will have an SpO₂ of 100%.⁹ Similarly, during a bleeding episode the PCV may not reflect the degree of blood loss unless IV fluids are also being administered. A change in total protein can also be used to estimate blood loss. The best method of determining the need for blood is monitoring blood pressure and perfusion parameters (heart, respiration and pulse rates, pulse quality, body temperature, CRT, mentation, lactate concentration).⁹ The administration of whole blood provides circulatory volume support and coagulation factors which may be beneficial in patients with bleeding disorders. Fresh whole blood also provides transient (three to four hours) benefit to patients with platelet diseases.

Expected blood loss during surgery

Where surgery is likely to result in significant blood loss, presurgical assessment and preparation for transfusion of the patient are essential. Cats can have significant anemia and show minimal clinical signs due to its chronicity. Through lifestyle adaptations they may appear normal to the owners despite an extremely low PCV. Although this chronic anemia is well tolerated, an acute blood loss can result in sudden deterioration in the patient’s ability to cope with the anemia. Prior to the surgical procedure the cat’s PCV and perfusion parameters (pulse quality, mucous membrane color and capillary refill time) must be assessed. Where anemia is moderate to severe (PCV <20%) it should be corrected with transfusion prior to anesthesia. In cats with mild-moderate anemia (PCV >20%), arrangements should be made to have blood available if blood loss during surgery is predicted.

Stabilizing an anemic patient prior to surgery

First, the cause of the anemia must be determined. Patients with chronic blood loss or those with non-regenerative anemia may gain significant long-term benefit from transfusion. Transfusion can stabilize the patient for two to three months as transfused red blood cells can have a near normal life span (70 days). Prior to anesthesia a PCV of at least 20% should be achieved.

Cats with hemolytic anemia receive transient benefits from a transfusion and should have their hemolytic disease stabilized and under long-term control prior to surgery whenever possible.

Note that clotting factors, platelets and the anti-inflammatory/immunoregulatory properties of fresh whole blood last only two weeks in human blood units and it is likely that this is much shorter in cats.⁹

Disorders of secondary hemostasis

The most common cause of clotting problems is vitamin K deficiency, which can be caused by anorexia or hepatic disease involving the biliary tract. Vitamin K is a fat-soluble vitamin with a relatively short half-life that requires bile acids for its uptake. Cats can rapidly become deficient in the vitamin leading to coagulation disturbances similar to those seen in coumarin toxicity.10,11 All cats with significant anorexia and those suspected of having significant biliary disease should be given vitamin K by injection 24 hours prior to surgery. Oral vitamin K will not aid those patients with compromised vitamin K uptake. In cats with disseminated intravascular coagulation, whole blood will provide clotting factors and antithrombin. However, if the patient is not anemic then fresh frozen plasma, if available, will suffice.^6,7 Further discussion on coagulopathies in cats is given in Chapter 1.

Feline blood groups

Two major blood group systems have now been identified in cats. The most important is the AB system which has three types (A, B and AB).12,13 Type A is the most common, whereas type B proportions vary both geographically and with breed.¹⁴ Type AB cats are consistently rare and only found in breeds with type B cats (20). The prevalence of type B can be very high (approaching 50%) in some pedigree breeds, especially the British shorthair, ragdoll, Birman, and Rex breeds. Type B cats are also commonly encountered amongst Persians, Somalis, Abyssinians, Himalayans, and Scottish folds. However, notable geographic and individual variation in blood type can occur within these breeds.^14–20 It appears that feline blood types are well conserved as wild felids have the same blood groups as domesticated cats.²¹

In addition, cats have naturally occurring alloantibodies against the blood type antigen that they do not possess.22,23 These alloantibodies result from exposure to antigenic epitopes commonly found in nature that are similar or identical to ‘non-self’ blood group antigens. Therefore, when a cat with naturally formed alloantibodies encounters ‘foreign’ type red blood cells a transfusion reaction ensues. All type B cats have high levels of naturally occurring anti-A antibodies. Many type A cats (>70%) have low levels of naturally occurring anti-B antibodies but this varies geographically.²³ Type AB cats do not have naturally occurring alloantibodies.^13,23

The Mik red cell antigen was recently reported. Cats whose red cells do not express the Mik cell antigen can have naturally occurring alloantibodies against the Mik antigen (approximately 4% of cats in the study).²⁴

Blood typing and cross-matching techniques

Cats must receive blood of the same type as their own or significant and potentially fatal transfusion reactions can occur.25,26 These reactions may be either immediate hemolysis or delayed (up to five days later). Compatibility can only be guaranteed if donor and recipient are the same blood type and have a negative cross-match. Cross-matching alone does not always ensure that donor and recipient are the same blood type.

A variety of desktop blood typing kits are available. In-house systems use very small volumes of blood and positive results are seen as an agglutination reaction (Rapid Vet-H), or a test line (Quick Test A+B) (Fig. 5-1). Many diagnostic laboratories will also perform these typing tests. However, blood typing will not identify incompatibilities associated with previous transfusions, which may not be AB associated. Therefore blood typing AND cross-matching (Fig. 5-2) should ideally be performed at the first transfusion and must be performed before subsequent transfusions.²⁷ How to perform a cross-match is summarized in Box 5-2.

Box 5-2 Performing a cross-match