The direct Coombs test is used to determine whether red blood cell (RBC)-binding antibody (IgG) or complement (C3) is present on RBC membranes. The patient's RBCs are incubated with antibodies to human IgG and C3. If IgG or C3 is bound to RBC membranes, agglutination occurs–a positive result. A positive result suggests the presence of autoantibodies to the patient's RBCs. If the patient has received a transfusion in the last 3 months, a positive result could also represent alloantibodies to transfused RBCs (usually occurring in acute or delayed hemolytic reaction).