The direct Coombs test detects antibodies on the surface of the red blood cell that are capable of destroying the cell (hemolysis) and causing anemia. There are several antigens on the surface of red blood cells, including those based on the blood groups (A, B) and Rh factor. In addition to blood group incompatibility between a mother and her baby and blood transfusion reactions, several conditions can lead to the formation of antibodies against these surface antigens, such as autoimmune diseases, lymphoma, leukemia, infections and certain drugs administered intravenously (penicillin, cephalosporin, piperacillin).
A positive result for direct Coombs means that antibodies are attached to the red blood cell surface, but it does not indicate the type of antibody present or whether it may cause symptoms. The medical history of the case and a clinical examination are necessary to determine the cause of the presence of these antibodies (transfusion reaction, autoimmune disease, infection, mother–fetus blood type incompatibility). A small percentage of the normal population is positive for direct Coombs but does not suffer from hemolytic anemia. A negative direct Coombs test indicates that antibodies are not likely to be attached to the red blood cell surface and that the reported signs and symptoms are due to another cause.