Anemia is commonly seen in veterinary emergency and critical care medicine. Patients may be brought in with the presenting
complaint of anemia or may develop anemia during hospitalization as a result of their underlying disease or treatment. Anemia
may contribute to patient morbidity, cost of treatment, and length of stay, frequently necessitating expensive interventions
such as blood transfusion while the underlying disease is being treated.
Classification of anemia
Anemia seen in veterinary patients may be classified into three broad categories that relate to cause; blood loss, hemolysis,
and decreased production. Classification of anemia in this way is not merely academic but is crucial to the workup of anemic
patients. Because regenerative and non-regenerative anemias have different sets of differentials and diagnostics, this classification
will guide further testing and provide useful prognostic information.
Three simple, in-house diagnostic tests can be performed in all anemic patients to help classify their anemia. These tests
are inexpensive, easy to perform, and will frequently provide a great deal of information about the cause of the anemia. They
can all be performed in approximately 5 minutes, allowing the clinician to classify the anemia and provide an appropriate
diagnostic plan while the owner is still present.
The first test is the packed cell volume (PCV) and total solids (TS). The importance of interpreting the PCV in conjunction
with the total solids cannot be overemphasized. If the PCV and TS are both low, acute blood loss should be suspected. In contrast,
a low PCV with normal total solids would be consistent with hemolysis or decreased red blood cell production. To differentiate
these two clinical entities, the plasma of the spun sample should be carefully evaluated for the presence of hemoglobin or
bilirubin that may suggest intravascular or extravascular hemolysis respectively.
The second test that should be performed is the blood smear. Blood smears are useful for differentiating hemolysis from decreased
production anemia as the presence of significant polychromasia and anisocytosis may indicate the presence of a regenerative
response. Blood smears should also be evaluated for blood parasites and telltale alterations in red blood cell morphology.
Heinz bodies are characterized by bulging of the red blood cell membranes and indicate oxidative red blood cell damage secondary
to toxins such as onions, garlic, or propylene glycol. Spherocytes are small, round erythrocytes with loss of central pallor
that result when antibodies bound to red blood cell membranes lead to a portion of the membrane being phagocytized or "pinched
off" by macrophages. Large numbers of these cells are typically seen in dogs with immune-mediated hemolysis. Schistocytes
are erythrocytes that have become fragmented as a result of passage through narrowed microvasculature. Schistocytes typically
reflect microangiopathic causes of hemolysis such as caval syndrome, disseminated intravascular coagulation, hemangiosarcoma,
or splenic torsion. Acanthocytes are red blood cells with long spiny projections that are frequently seen in patients with
hepatic or splenic neoplasia, though they may also be seen in animals with disorders of lipid metabolism as well.
Finally, a slide agglutination test should be performed when hemolysis is suspected. In this test, a drop of anticoagulated
blood from a purple top tube or capillary tube is mixed with several drops of saline. Autoagglutination may be evidenced by
the development of obvious flecks within the drop of blood. The saline is used to disperse rouleaux that may mimic agglutination.
Autoagglutination is caused by cross-linking of antibodies bound to the erythrocyte membranes, and as such is diagnostic for
an immune-mediated component to the hemolysis.
Formulating a list of differential diagnoses
Once the anemia has been classified as blood loss, hemolysis, or decreased production, a list of differentials may be formulated.
(see table 1)
Table 1. Some common differentials for anemia