Immune-Mediated Hemolytic Anemia, Canine - contd.
Anemia is diagnosed by documenting the presence of decreased red blood cells. This is most commonly done in the hospital with measurement of a packed cell volume, or PCV. Measurement of a hematocrit is usually done in a reference laboratory. Once anemia has been documented there are many tests that are useful in further classifying the anemia.
Anemias may be classified as regenerative and nonregenerative. Regenerative anemias arise from factors that generally do not suppress the bone marrow, the normal source of red blood cells. Nonregenerative anemias occur as a result of bone marrow-suppressing events. Reticulocytes are immature, developing red blood cells. A reticulocyte count tells how many immature red blood cells are in circulation. Most dogs with immune-mediated hemolytic anemia show a regenerative response, with increased reticulocyte counts or large numbers of nucleated red blood cells in circulation. The most common causes of regenerative anemias are blood loss and hemolysis, or red blood cell breakdown. Hemolysis should be suspected if there is a regenerative anemia with no evidence of internal or external blood loss. Not all forms of immune-mediated anemia are associated with increased numbers of reticulocytes. When the immune-mediated injury is directed at cells in the marrow, the reticulocyte count is decreased. This form of immune-mediated, non-regenerative anemia, is sometimes referred to as pure red cell aplasia.
Additional red blood cell features are helpful in classifying anemia as regenerative or nonregenerative. Red blood cell size is often increased and color is often less intense than normal in regenerative anemias. Nucleated red blood cells, which are released early from the bone marrow when demand is high, are often elevated in regenerative anemia. The most specific red blood cell change in IMHA is the formation of spherocytes. A spherocyte is a round-looking red blood cell that lacks the typical zone of paleness in its center when examined on a blood smear. It is thought that spherocytes are only seen with immune-mediated injury.
Red blood cell agglutination, or clumping, may be noted when blood is collected from dogs with IMHA. The presence of agglutination is usually thought to be specific for immune-mediated anemia, but its absence does not rule it out. Agglutination may be either macroscopic, which means visible to the naked eye, or microscopic, indicating it can only be noted when drops of blood are examined under a microscope.
When IMHA is suspected, a direct antibody, or Coombs test is usually performed. This test looks for antibodies against red blood cells. Although many dogs with IMHA will have a positive Coombs test, some do not. A negative Coombs test does not rule out the possibility of IMHA. Coombs test results should always be interpreted in light of other clinical and laboratory findings.
Serum biochemical profile abnormalities are common in IMHA, but none of the commonly seen changes are specific for the diagnosis. Serum bilirubin levels are increased due to excessive red blood cell breakdown. Liver enzymes may be elevated; this occurs when anemia causes the liver to receive a decreased amount of oxygen. Protein levels are usually normal to increased in hemolytic anemia. This is a key point in distinguishing hemolysis from blood loss, since protein levels are usually decreased in whole blood loss situations.
Changes in the white blood cell and platelet counts may be seen in addition to red blood cell count abnormalities on a complete blood count. Some dogs with IMHA have marked increases in their white blood cell counts. This occurs when all cell lines within the bone marrow are excessively stimulated. Other dogs with IMHA may have decreased platelet counts, particularly if the immune-mediated injury involves platelets as well as red blood cells. This is called Evan's syndrome, and may be associated with a worse outcome in most patients than with IMHA alone.
A complete blood count will also help reveal underlying causes for hemolytic anemia. Red blood cell parasites that can cause hemolysis may sometimes be seen on evaluation of a blood smear. Although rarely seen, Heinz body formation in dogs can occur from onion ingestion or from acetaminophen overdose. Heinz bodies are structures composed of denatured hemoglobin that can also be seen in some dogs with hemolytic anemia.
Other diagnostic tests may also help identify an underlying cause for hemolytic anemia. These tests may help differentially diagnose IMHA from non-immune-related hemolytic anemias. Chest and abdominal x-rays and abdominal ultrasound may be used to screen for evidence of cancer in older dogs with suspected IMHA. Cancer, especially lymphosarcoma, is often associated with immune-system abnormalities. Abdominal x-rays may also be useful in identifying zinc-containing foreign objects like coins, which can induce hemolytic anemia in dogs. Tests for tick-borne infectious diseases may need to be considered as well. Geography influences the incidence of such diseases; many tick-borne diseases occur more commonly in the southern and the southeastern parts of the United States. Bone marrow evaluation may be recommended if the CBC reveals unusually low platelet and white blood cell counts, or if the anemia is nonregenerative. This is done primarily to rule out the presence of diseases like leukemia within the bone marrow itself, and to further document the marrow's ability to respond to the demand for new blood cells.
When IMHA has been tentatively diagnosed, when no likely underlying cause has been identified by additional testing, and when there is no history of recent drug or vaccine exposure that may have triggered the event, the condition is often referred to as autoimmune hemolytic anemia, or AIHA. It is assumed in these cases that for unknown reasons, the immune system targets otherwise normal red blood cells, and tries to remove them from circulation.
