Anemia (PCV < 38% in the dog and < 24% in the cat) is frequently seen in clinical practice and can be grouped into 3 broad categories to aid in diagnosis of the underlying cause: blood loss, hemolysis or decreased production. Classifying anemia in this way can guide testing and suggest prognosis.
There are three, simple diagnostic tests that can be performed in-house which can quickly help the veterinarian classify the anemia and make a diagnostic plan. They are inexpensive and can be completed in just a few minutes. The tests are PCV (packed cell volume) and TS (total solids), a blood smear and a slide agglutination test.
The first test, a PCV and TS, need to be interpreted together. A low PCV and low TS suggests blood loss although in acute blood loss the PCV and TS will initially be normal and then drop (first the TS in 1-4 hrs then the PCV in 12-24 hrs). A low PCV combined with a normal TS could be consistent with either hemolysis or decreased RBC production. Examining the plasma in the hematocrit tube and finding hemoglobin or bilirubin would be suggestive of either intravascular or extravascular hemolysis.
The second test, which is evaluating a blood smear, can differentiate hemolysis from decreased production. Significant polychromasia and anisocytosis can represent a regenerative anemia. Blood parasites (Mycoplasma haemofelis, M. haemominutium, cytauxzoonosis, babesiosis), changes in red blood cell morphology and Heinz bodies can also be identified on a blood smear. Heinz bodies represent denatured and precipitated hemoglobin from oxidative injury caused by toxins such as onions, garlic, methylene blue and Vitamin K3. Cats are more susceptible to Heinz body formation and can also be seen with hyperthyroidism, lymphoma, other cancers and diabetes mellitus (esp. ketoacidosis). Spherocytes (small, round RBCs with no central pallor) can be seen with IMHA. Schistocytes (fragments of RBCs) represent microangiopathic causes of hemolysis such as caval syndrome, DIC, hemangiosarcoma or splenic torsion. Acanthocytes (RBCs with spiney projections) can be seen with hepatic or splenic neoplasia or disorders of lipid metabolism.
The third test is a slide agglutination test which is performed when hemolysis is suspected. A drop of anticoagulated blood from the purple top tube or capillary tube is mixed with several drops of saline and is examined for autoagglutination. Autoagglutination is diagnostic for IMHA and is caused by the cross- linking of antibodies on the RBC membrane. A Coombs test is NOT necessary if there is autoagglutination.
To further diagnose blood loss requires a thorough history and physical exam, a minimum database (CBC, biochemistry panel, urinalysis), platelet count, reticulocyte count, coagulation profile and imaging (radiographs and/or ultrasound). Differentials for blood loss include trauma, neoplasia, primary hemostatic defect (thrombocytopenia, thrombocytopathia, von Willebrands), secondary hemostatic defects (rodenticide toxicity, hemophilia, DIC), GI losses or surgical/iatrogenic losses.
Hemolysis can be immunologic or non-immunologic. Testing can include a minimum database, platelet count, reticulocyte count, testing for vector-borne illnesses, FeLV/FIV testing, imaging and possibly a Coombs test if hemolysis is suspected but there is no autoagglutination (not a sensitive test). Hemolytic anemias are usually highly regenerative but it takes 3-5 days to get a response. Immunologic hemolytic anemias can be primary or secondary. Secondary IMHA may be triggered by vaccinations (w/in 4 weeks), drugs (sulfas, penicillins, cephalosporins), neoplasia, vector-borne diseases (Ehrlichiosis, Babesiosis, Bartonellosis, Hemoplasmosis) and FeLV/FIV. Non-immunologic hemolysis can be caused by zinc exposure, oxidative toxins, microangiopathic conditions, hypophosphatemia, hereditary diseases and others.
Decreased production anemias often require bone marrow histopathology to characterize the anemia and are classically non-regenerative. They can include anemia of chronic disease, chronic renal failure, endocrine diseases (hypothyroidism, hypoadrenocorticism), pure red cell aplasia, generalized bone marrow hypoplasia (from radiation, toxins, drugs, infectious agents, myelophthisis, myelofibrosis), iron deficiency and myelodysplasia.