What is a Coombs test?
A Coombs test is used to test for a disease called immune-mediated hemolytic anemia (IMHA). IMHA is a condition in which the immune system breaks down or destroys red blood cells, leading to anemia.
A Coombs (or direct antiglobulin) test detects the presence of immunoglobulins (antibodies) on the surface of red blood cells. Immunoglobulins are proteins made by white blood cells (specifically plasma cells).
The Coombs test detects these immunoglobulins using antiserum that reacts against different types of immunoglobulins. If this antiserum detects immunoglobulins on the surface of the red blood cells, it causes the red blood cells to clump together (agglutinate) in the test tube. For this test, a single blood sample is sent to a veterinary reference laboratory.
What is immune-mediated hemolytic anemia (IMHA)?
Immune-mediated hemolytic anemia is one of the most prevalent immune-mediated disorders in dogs and cats. It is a condition in which the pet’s immune system attacks its own red blood cells as something that should be eliminated or destroyed (like how an immune system would attack an invading virus). The immune system “targets” the red blood cells by coating them with immunoglobulins (antibodies). Coated red blood cells are destroyed (hemolysed) by specialized white blood cells (macrophages) either within the blood stream or within the spleen or liver.
"The causes of IMHA can generally be divided into two categories:
primary (also called idiopathic) and secondary."
The causes of IMHA can generally be divided into two categories: primary (also called idiopathic) and secondary. With primary IMHA, the underlying reason that antibodies are produced against an animal's red blood cells remains unclear or unknown. With secondary IMHA, the production of antibodies against red blood cells is secondary to another condition such as infection, allergy, inflammation, or cancer (benign or malignant tumors). Certain drugs are known to trigger IMHA in susceptible animals; however, the effects of these drugs on individual animals cannot be predicted.
Are there any other ways to diagnose immune-mediated hemolytic anemia?
IMHA will be suspected in any animal that has clinical signs such as pale gums or unexplained weakness, and characteristic laboratory evidence of immune-mediated red blood cell destruction.
Laboratory evidence of immune-mediated red blood cell destruction includes mild to severe anemia accompanied by strong evidence of new red blood cell production (to keep up with the destruction). The presence and degree of anemia is determined by a complete blood count (CBC). Assessment of a blood smear (often included in the CBC) can identify increased numbers of young red blood cells, indicating increased production from the bone marrow. The young red blood cells are called polychromatic erythrocytes or, if a special stain is used, reticulocytes.
Some red blood cells are not destroyed during IMHA, but only damaged. Spherocytes are abnormally shaped red blood cells that have had a portion of their membrane removed by the immune cells because of the abnormal presence of antibodies in IMHA. High numbers of spherocytes can indicate IMHA.
Another sign of IMHA is auto-agglutination of red blood cells. The presence of antibodies on the surface of red blood cells makes them “sticky”, sometimes causing them to clump together on the blood smear. A serum chemistry profile may indicate an increase in bilirubin. Bilirubin is a compound formed in the liver due to the breakdown of red blood cells. Normally, bilirubin is found in small amounts in a serum sample, but with immune-mediated destruction of red blood cells, the amount of bilirubin will increase.
Taken together, the physical findings and the laboratory data may suggest that immune-mediated destruction of red blood cells is the most likely cause of your pet's anemia. The Coombs test may provide further confirmation of the presence of immunoglobulin on the red blood cell surface.
What does a positive test result mean?
A positive Coombs test means that the red blood cells are coated with hundreds of immunoglobulins. A positive result is supportive of immune-mediated destruction of red blood cells when considered with other supporting laboratory data and clinical signs. However, a positive Coombs test does not allow your veterinarian to determine if the red blood cell destruction is due to primary or secondary immune-mediated hemolysis.
Positive Coombs test results can sometimes occur in animals without immune-mediated hemolytic anemia, so it is important to determine whether the CBC and serum biochemistry panels contain supportive evidence for immune-mediated red blood cell destruction.
What does a negative test result mean?
A negative Coombs test can indicate that your pet does not have IMHA. However, approximately 10% to 30% of dogs with IMHA will have a false negative Coombs test result. False negatives occur because red blood cell hemolysis can occur with as few as 20 to 30 molecules of antibody attached to the red blood cell, whereas the Coombs test requires the presence of more than 200 to 300 molecules before a positive result is noted.
Recent treatment with steroids may also cause a Coombs test to be falsely negative.
