In addition to blood typing, cross-matching can identify if there is compatibility or incompatibility between a blood donor and recipient. It tests for the presence of alloantibodies, induced or naturally occurring, that ­cannot be detected with blood typing. Traditionally, blood typing has been recommended before the first transfusion and cross-matching only before a second transfusion if performed more than 4 days later as development of alloantibodies is expected. There is evidence that naturally occurring antibodies other than the AB blood group occur in cats. In a retrospective study of 300 cats that received a blood transfusion, major cross-match incompatibilities were documented in 15% of transfusion-naïve cats and in 27% of previously transfused cats.3 The prevalence of naturally occurring non-AB incompatibilities may be justification for performing cross-matching before all transfusions in cats although the clinical significance of these alloantibodies remains to be determined. In a smaller study of 48 cats receiving a first transfusion, there was no significant difference in the incidence of transfusion reactions between cross-matched and non–cross-matched transfusions.⁴ It is worth noting that the prevalence of non-AB incompatibilities seems to be more common in the United States than Europe. Studies have found non-AB incompatibilities to be rare in cats in the United Kingdom⁵ and Germany.^6,7

One study has shown administering type- and cross-match–compatible transfusions was associated with a greater increase in packed cell volume (PCV) post-transfusion than use of type-specific, non–cross-matched blood.⁸ However, in two studies, there was no significant difference in the mean change in PCV after transfusion with cross-matched versus non–cross-matched blood.^3,4 In one of the studies, febrile transfusion reactions were more common in cats receiving type-specific blood without a cross-match.³

The major cross-match tests for alloantibodies in the plasma of the recipient that may hemolyze the donor’s red cells. The minor cross-match tests for alloantibodies in the plasma of the donor that may attack the recipient’s red cells. Autoagglutination in the major cross-match predicts that antibodies in the recipient’s plasma will attack the donor’s red cells, and a minor cross-match incompatibility suggests that antibodies in the donor’s blood will attack the recipient’s red cells. Incompatible blood may cause a transfusion reaction.

A quick method for performing a major cross-match is to mix two drops of plasma from the recipient with one drop of anticoagulated blood from the donor on a slide at room temperature. Separate pipettes or syringes must be used for all solutions. The opposite will be a minor cross-match. Development of macroscopic agglutination within one minute suggests the presence of anti-A alloantibodies in the recipient (major cross-match) or donor (minor cross-match). In both cases, the blood is incompatible. Autoagglutination can make interpretation of the test difficult. Running a control test using saline instead of plasma may help with interpretation. Macroscopic agglutination should be graded as follows:⁹

For hospitals performing frequent transfusions, standardized gel agglutination kits (e.g., DMS Laboratories, United States; Alvedia, France) and immunochromatographic kits (e.g., Alvedia, France) are available for patient-side use. Although more time consuming than the previously described method, the gel test is less vulnerable to error. Because it is stable, the test result can be saved and reviewed later if needed. More rigorous and time-consuming methods involving washing, centrifuging, and incubating samples have been described.9

A positive, properly performed slide agglutination test suggests the presence of antibody-coated erythrocytes and negates the need for performing a direct Coombs’ test (direct antiglobulin test). It is important to differentiate erythrocyte clumping caused by autoagglutination from that caused by rouleaux formation. These types of erythrocyte clumping are differentiated by washing the cells with saline, which will break up clumps formed by rouleaux. A quick method of performing the test is to mix a drop of EDTA anticoagulated blood on a slide with two to five drops of 0.9% NaCl, followed by gross and microscopic examination of the sample. The “stacked coins” appearance characteristic of rouleaux formation (Fig. 28.4) will disperse, whereas the random or rosette clumping of autoagglutination will not (Fig. 28.5). If the test is negative, a direct Coombs’ test should be requested. An important limitation to this test is its inability to separate primary from secondary immune-mediated disease.

Erythrocytes can be classified by size and hemoglobin concentration based on quantitative parameters such as the mean corpuscular volume (MCV; the average cell size), the red cell distribution width (RDW), and the mean corpuscular hemoglobin concentration (MCHC). Macrocytosis, normocytosis, and microcytosis refer to cell size above, within, or below the reference range, respectively. The RDW is derived from the cell numbers versus cell volume histogram (Fig. 28.7); an increase in RDW indicates a greater than normal variation in cell size. The RDW may be artifactually affected by the overlap in size between feline platelets and red cells.

Qualitative erythrocyte parameters are based on a blood smear evaluation. Increased variation in cell size, color, and shape are known as anisocytosis, polychromasia, and poikilocytosis, respectively. Anisocytosis is present if there is a combination of cells of normal size along with an appreciable number of larger or smaller cells. Anisocytosis may result in an increased RDW. The larger cells are often reticulocytes, although infection with feline leukemia virus (FeLV) can result in larger cells without increased reticulocyte numbers. Polychromasia is usually due to the presence of increased numbers of aggregate reticulocytes and indicates regeneration. Lack of polychromasia, however, does not rule out regeneration. Variations in cell shape may be artifactual or due to disease (Fig. 28.8). Echinocytes are crenated red cells with uniform, often pointy, projections. They are usually artifacts but are important to recognize; when the projections are viewed end on, they may appear as small rings and mimic the ring form of hemoplasmosis (e.g., Mycoplasma haemofelis). Acanthocytes look like echinocytes but have fewer and more rounded projections. They are frequently present in cats with hepatic disease. Erythrocyte fragments, such as schistocytes or keratocytes, are the result of cell trauma. When there are many fragments, the presence of turbulent blood flow or microangiopathic disorders such as hemangiosarcoma or disseminated intravascular coagulation (DIC) should be considered. Iron deficiency may also cause increased fragmentation. Spherocytes are smaller cells that are the product of immune-mediated removal of antibody-coated parts of the erythrocyte membrane, after which the cell is reconfigured into a sphere. Because normal feline erythrocytes are small and lack central pallor, spherocytosis is difficult or impossible to appreciate and identification is best left to an experienced veterinary hemocytologist.

Microagglutination and rouleaux formation may be visible on blood smears. Agglutination appears as a random, disorganized clumping of cells not dispersed by the addition of saline. True autoagglutination indicates an immune-mediated disease affecting the erythrocyte. Rouleaux formation looks like a stack of coins (Fig. 28.4) and will disperse after the addition of saline (Fig. 28.5). Circulating monocytes may phagocytose antibody-covered red cells; this is called erythrophagocytosis. Although not observed very often, erythrophagocytosis also suggests the presence of immune-mediated red cell damage.

Heinz bodies are areas of oxidatively denatured hemoglobin within the cell (discussed later). The altered hemoglobin is pushed to one side and is often seen as a projection from the surface of the cell membrane. Heinz bodies stain darkly with NMB stain, somewhat clear with Diff-Quik stain (a variation of Romanowski stain), and the same as the cytoplasm with Wright’s stain (Fig. 28.9). Howell–Jolly bodies are intracytoplasmic remnants of nuclear material found in erythrocytes that may mimic red cell parasites. Blood smear examination is an essential part of the CBC, particularly for evaluating the erythron. There is no other way to identify the morphologic changes in red cells that can yield clues to the etiology of erythrocyte disease. Without a blood smear evaluation, a CBC is incomplete.

The cat has one major blood group system with three common serotypes: A, B, and AB (sometimes called type C). The blood types are due to genetically determined erythrocyte surface antigens. The antigens are sialic acids produced by the enzyme cytidine monophospho-N-acetylneuraminic acid hydroxylase. N-glycolylneuraminic acid is associated with blood type A and N-acetylneuraminic acid with type B. The A-allele is dominant over the b-allele so that cats with genotypes A/A and A/b will be type A, whereas only cats with homozygous b/b will have the type B phenotype. A third blood type, AB, occurs rarely and controversy exists regarding inheritance as it appears different cat breeds have different inheritance patterns. Cats with blood type AB have both antigens (N-glycolylneuraminic acid and N-acetylneuraminic acid) expressed on red cells. A and B antigens are produced on the same red cell only in cats with Ab/b or Ab/Ab genotypes. Another potentially important type called Mik was identified in 2007.10 A more in-depth discussion of feline blood types has been published and can be consulted for more information.¹¹

Several methods for blood typing are available such as card, slide, and tube agglutination tests. Immunochromatographic cartridge test kits are also available; they are not affected by rouleaux formation or autoagglutination and may have better performance than typing cards.^12,13 Blood typing can be performed by a diagnostic laboratory or patient-side with a test kit (e.g., DMS Laboratories, United States; Alvedia, France). The strength of the reaction should be graded. If a patient-side test is used, type AB and type B results should be confirmed by a referral laboratory because some cross-reactions have been known to occur.¹⁴ As well, any weak reactions, autoagglutination, or results in cats with FeLV infection should be confirmed.

Genetic blood typing using buccal mucosal swabs or blood samples is available from some commercial laboratories and may allow breeders to identify heterozygous type A cats (A/b). Breeding two such cats together is expected to produce 25% type B kittens (b/b) and 50% heterozygous type A kittens (A/b). Genetic testing at commercial laboratories is also available to identify type AB cats.

Understanding feline blood groups is important because, unlike other mammals, cats produce naturally occurring antibodies (alloantibodies) against erythrocyte antigens that are not present on their own cells. Kittens start producing alloantibodies at approximately 2 to 3 months of age as a result of exposure to antigens on plants, bacteria, or protozoa that are structurally similar to red cell antigens. No alloantibodies are produced against antigens that are similar to self-antigens, and no previous exposure to blood products (e.g., transfusions) is necessary to produce the alloantibodies.

Knowledge of this system is important in the prevention of transfusion reactions and neonatal isoerythrolysis (NI). Cats with type B blood have anti-A antibodies with strong hemolytic potential. As little as 0.5 mL of type A or AB blood administered to a type B cat can cause potentially life-threatening hemolysis within minutes of the transfusion; in addition, the transfused erythrocytes have a lifespan of only about 1 day.¹⁵ Hemolysis of type B blood administered to a type A cat will result in reduced life span (about 2 days) for the transfused erythrocytes although severe reactions are uncommon. Type AB cats are compatible with type A or type AB whole blood.

The distribution of blood types varies by geographic region and breed. Type A is the most common feline blood type. There is, however, geographic variation in the number of type B domestic shorthair cats. More than 10% of the domestic shorthair cats in Australia, France, Greece, India, Ireland, Italy, Japan, Turkey, Israel, New Zealand, the west coast of North America, and some regions of England are type B. Distribution of blood types among pedigreed breeds does not vary as much by location because of the international exchange of breeding cats. More than 30% of British Shorthair, Cornish and Devon Rex, and Turkish Angora and Van cats have type B blood. The Abyssinian, Birman, Himalayan, Somali, Persian, and Scottish Fold breeds are 10%–25% type B. In contrast, Siamese and related breeds are almost exclusively type A. Ragdoll cats appear to be unique as this is the most common breed with blood type AB. Although 31 of 213 mixed-breed cats in Israel were type AB,¹⁶ the AB blood type seems to be rare in the rest of the world, and the frequency of the Mik blood type is unknown. The presence of erythrocyte antigens in addition to the A and B groups may explain why transfusion compatibility is not guaranteed by blood typing alone; cross-matching is recommended before all transfusions. Breeding queens, along with blood donors and, if possible, transfusion recipients, should be blood typed.

Each blood product has specific uses. Fresh whole blood contains erythrocytes, platelets, clotting factors, and serum proteins. Storage of whole blood results in the loss of platelet clotting factors V and VIII, although vitamin K-dependent clotting factors are stable.9 Packed red cells maintain the oxygen-carrying capacity of whole blood in a smaller volume. This product may be used when volume expansion is unwanted, such as anemic cats with heart disease. Fresh frozen plasma contains albumin and all the clotting factors and is used in cats hemorrhaging from coagulation disorders such as liver failure, DIC, or anticoagulant rodenticide toxicity. The use of plasma products to treat hypoalbuminemia is beneficial only in the short-term because transfused albumin rapidly equilibrates with the extravascular space. The addition of synthetic colloids may prolong the oncotic effects of a plasma transfusion in these cats. Platelet-rich plasma is indicated for cats bleeding from platelet deficiency or dysfunction. Oxygen-carrying solutions based on bovine hemoglobin have been available for veterinary use in the past; while not available as of this writing, new products may become available for human and veterinary use.

Blood donor cats should be healthy, fully vaccinated, large cats with a PCV >35%. Donors should be blood typed and a CBC should be performed before blood is collected. No abnormal morphologic cell types should be present, and platelet numbers should be within the reference range. Several guideline documents have been published on screening blood donors to reduce the risk of infectious disease transmission ( e-Box 28.1). For example, the American College of Veterinary Internal Medicine (ACVIM) consensus statement on screening blood donors recommends testing donor cats for M. haemofelis, Candidatus Mycoplasma haemominutum, FeLV antigen, feline immunodeficiency virus (FIV) antibody, and, possibly, Bartonella infections.¹⁷ Experts also recommend PCR testing for FeLV to detect infected cats that are not identified with antigen tests alone.^18,19 Cats that test positive for FIV antibody should be excluded even if vaccinated against the disease because discriminating antibodies due to natural infection from vaccination can be difficult. Heartworm disease cannot be transmitted by blood donation as the larvae require passage through the mosquito to become infective. Screening for cytauxzoonosis is unnecessary because most cats with the disease are clinically ill. Toxoplasmosis and feline infectious peritonitis are not known to be transmitted by transfusion.¹⁷ Donor cats should be kept indoors to reduce the risk of exposure to infectious diseases.

Healthy cats can donate 10% to 20% of their total blood volume without adverse effects. A cat’s total blood volume is approximately 66 mL/kg. For example, approximately 50 mL of blood (10 mL/kg) can be collected from a 5-kg cat no more often than every 4 to 6 weeks. Subcutaneous fluids should be administered at two to three times the volume of donated blood. Collection of more than 70 mL of blood from a 5 kg cat can lead to hypovolemia, and the volume should be replaced with IV fluids. Many donors resent sitting still long enough to have this volume of blood removed and may require sedation or general anesthesia.

Whole blood collected for immediate use can be anticoagulated with heparin. Heparin has no preservative properties so heparinized blood should be used within 24 hours if stored at room temperature and within 48 hours if refrigerated.⁹ If whole blood is to be stored for a longer period (up to 3 weeks), citrated anticoagulants should be used and blood should be stored in 60 mL units. Other types of blood products are usually purchased from a blood bank, as most veterinary hospitals lack the specialized equipment necessary for processing.

For treatment of anemia, there is no established PCV level that dictates when a cat requires a blood transfusion. The decision to transfuse is based on the patient’s condition and assessment of the potential benefits and risks. Indications that an anemic cat may require a transfusion include respiratory distress, weak pulses, or severe weakness. For immune-mediated hemolytic anemia (IMHA), a common criterion is a PCV of 10% to 15% and for chronic non-regenerative anemia, a PCV of 8% to 12%. For acute blood loss, a common criterion for red cell transfusion is a PCV of 15% to 20%. However, these are guidelines only and should be interpreted considering the patient’s clinical signs, anticipated blood loss, and other parameters. The patient’s clinical status should be given more weight than any PCV-based criteria for transfusion.

Both the donor and recipient should be blood typed. Even if the blood types are known, cross-matching should be performed before blood administration to prevent incompatible transfusions caused by untested or unknown erythrocyte antigens such as Mik. The half-life of appropriately matched RBCs in the cat is 29 to 39 days, but for mismatched transfusions the half-life may only be a matter of hours. When type B blood is transfused to a type A cat, the life span of transfused RBCs is only 2 days. When type A blood is transfused to a type B cat, in addition to a potentially severe or fatal reaction, the lifespan of the transfused cells is only a few hours.

Multiple transfusions (whole blood or packed RBCs) are well tolerated in cats and may be critical for the survival of some severely ill patients.²⁰ A cross-match should be performed again whether blood is being transfused from the same donor or another donor is used. In a study of 21 anemic cats receiving a median of two whole blood transfusions, 25% apparently developed alloantibodies against erythrocyte antigens outside of the AB system as early as 2 days post-transfusion.²¹

The required blood volume for transfusion can be collected from the donor into a large syringe for administration. If refrigerated whole blood is used, it is not warmed before administration to preserve RBC viability and reduce the risk of bacterial growth. A room temperature blood transfusion may be necessary in some circumstances, such as when rapid administration is required, when chilled blood may cause arrhythmia, and when the recipient cat is hypothermic. In those cases, blood should be warmed to room temperature over 30 to 60 minutes or the transfusion line can be passed through an infusion warmer. Procedures such as immersing the blood bag in warm water or warming in a microwave are not recommended.

Blood products can be administered by the IV or intraosseous route, but intraperitoneal administration is not recommended. The smallest catheter size for whole blood transfusion in cats is 22 gauge. Bacterial contamination is a potential risk, and aseptic techniques for blood collection and administration should be followed. Hands should be washed, and care taken when handling transfusion lines.

Ideally, a blood transfusion is delivered using a special administration set that contains an in-line blood filter. Human pediatric sets can be used for cats or the blood can be placed in a burette, an in-line filter can be added, and the line can be piggy-backed into the regular IV fluid line. The same IV line should not be used to administer 5% dextrose (may cause clumping and hemolysis) or calcium-containing solutions such as lactated Ringer’s (may cause microcoagulation). Fluids such as normal saline and replacement solutions can be given concurrently using the same line and can also be used to dilute a transfusion. The transfusion is usually administered by gravity flow, although an infusion or syringe pump (Fig. 28.11) can be used if the manufacturer indicates it can be used for this purpose. Administration can also be accomplished by intermittent slow bolus injections.

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