The direct and indirect Coombs tests, also known as antiglobulin test (AGT), are blood tests used in immunohematology. The direct Coombs test detects antibodies that are stuck to the surface of the red blood cells.[1] Since these antibodies sometimes destroy red blood cells they can cause anemia; this test can help clarify the condition. The indirect Coombs test detects antibodies that are floating freely in the blood.[1] These antibodies could act against certain red blood cells; the test can be carried out to diagnose reactions to a blood transfusion.[1]
The direct Coombs test is used to test for autoimmune hemolytic anemia, a condition where the immune system breaks down red blood cells, leading to anemia. The direct Coombs test is used to detect antibodies or complement proteins attached to the surface of red blood cells. To perform the test, a blood sample is taken and the red blood cells are washed (removing the patient's plasma and unbound antibodies from the red blood cells) and then incubated with anti-human globulin ("Coombs reagent"). If the red cells then agglutinate, the test is positive, a visual indication that antibodies or complement proteins are bound to the surface of red blood cells and may be causing destruction of those cells.
The indirect Coombs test is used in prenatal testing of pregnant women and in testing prior to a blood transfusion. The test detects antibodies against foreign red blood cells. In this case, serum is extracted from a blood sample taken from the patient. The serum is incubated with foreign red blood cells of known antigenicity. Finally, anti-human globulin is added. If agglutination occurs, the indirect Coombs test is positive.[2]
^ abc"Coombs test: MedlinePlus Medical Encyclopedia". medlineplus.gov. Retrieved 2019-05-06.
^F. Rosen and R. Geha, Case Studies in Immunology, 4th ed., Garland Science, p.173.
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