The direct Coombs' test determines whether RBC-binding antibody (IgG) or complement (C3) is present on RBC membranes. It is primarily used to help determine if the cause of hemolytic anemia is due to antibodies attached to RBCs. Some people make antibodies directed against their own RBC antigens. These autoantibodies may be produced in autoimmune diseases and with some other conditions, such as lymphoma, chronic lymphocytic leukemia, mycoplasma pneumonia and mononucleosis. This test is also used to help diagnose hemolytic disease of the newborn due to an incompatibility between the blood types of a mother and baby.