What is a Coombs Test?

A Coombs test, which is also called an antiglobulin test, is a procedure which analyzes blood for the presence of antibodies which can attack normal red blood cells. There are two types of Coombs tests: direct and indirect. A direct test looks for antibodies which have already attached themselves to the red blood cells. An indirect test analyzes the serum surrounding red blood cells for antibodies which may attach themselves and cause future problems. A Coombs test may help diagnose anemia, mononucleosis, syphilis, or a host of other ailments.

In a direct Coombs test, the patient's red blood cells are washed, eliminating the human serum. Then lab technicians incubate antihuman antibodies and the washed cells in a test tube and watch for the agglutination, or clumping, of red blood cells. A physician, generally a hematologist, looks at the results to determine a positive or negative test result. In a positive test result, agglutination is present. This primarily indicates hemolysis, or the breaking down of red blood cells.

The indirect Coombs test looks not at the red blood cells, but at the serum surrounding them. It attempts to determine if there are antibodies present which may bind to red blood cells, but have not yet done so. This test is often used as a screening test before a blood transfusion occurs or for pregnant women during prenatal care.

Ordered much less frequently than a direct Coombs test, the indirect test is often used for specific reasons. In the case of a blood transfusion, it is necessary to test the donor's blood for the presence of antibodies which may be transferred to the patient. For pregnant women, the test is sometimes administered to find out if antibodies might be passed through the placenta to the unborn child.

Risks associated with administering a Coombs test are minimal, and limited primarily to risks associated with drawing blood in general, since every Coombs test begins with the drawing of blood from a patient through routine procedure. Risks include hematomas, excessive bleeding or light-headedness. In addition, there is a small risk of bacterial infection whenever the skin is punctured, so the site must be cleansed before the blood is drawn and allowed to heal properly afterward.

Abnormal, or positive, test results are not a perfect indication of immunological dysfunction. Up to 3% of people may test positive. Elderly patients are more likely to have an abnormal test result.