A Coombs test detects antibodies in blood that can attack red blood cells. There are two types: direct and indirect. The direct test looks for antibodies already attached to red blood cells, while the indirect test analyzes the serum surrounding red blood cells. The test can help diagnose anemia, mononucleosis, syphilis, and other ailments. The indirect test is used for specific reasons, such as screening blood before transfusions or testing pregnant women for antibodies that may have passed to the unborn baby. Risks are minimal, and abnormal results are not always an indication of immunological dysfunction.
A Coombs test, also called an antiglobulin test, is a procedure that tests blood for the presence of antibodies that can attack normal red blood cells. There are two types of Coombs tests: direct and indirect. A direct test looks for antibodies that have already attached themselves to red blood cells. An indirect test analyzes the serum surrounding red blood cells for antibodies that can attach themselves and cause future problems. A Coombs test can help diagnose anemia, mononucleosis, syphilis, or a host of other ailments.
In a direct Coombs test, the patient’s red blood cells are washed, removing the human serum. Then lab technicians incubate the anti-human antibodies and washed cells in a test tube and observe the agglutination, or clumping, of the red blood cells. A doctor, usually a hematologist, reviews the results to determine a positive or negative test result. In a positive test result, agglutination is present. This mainly indicates hemolysis or breakdown of red blood cells.
The indirect Coombs test does not examine red blood cells, but the serum that surrounds them. Attempts to determine if there are antibodies that can bind to red blood cells, but hasn’t yet. 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 blood for the presence of antibodies that can be transferred to the patient. For pregnant women, testing is sometimes done to find out whether the antibodies might have passed across the placenta to the unborn baby.
The risks associated with administering a Coombs test are minimal and limited primarily to the risks associated with drawing blood in general, as each Coombs test begins with drawing blood from a patient through routine procedure. Risks include bruising, excessive bleeding, or lightheadedness. Additionally, there is a small risk of bacterial infection whenever the skin is punctured, so the site must be cleaned before 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 can test positive. Elderly patients are more likely to have an abnormal test result.
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