Direct Antiglobulin (Direct Coombs) Test (DAT)

The direct antiglobulin (direct Coombs) test is used to determine whether IgG antibody or complement (C3d) is present on red blood cell (RBC) membranes. The patient's RBCs are incubated with antibodies to human IgG and C3d. If IgG or C3d is bound to RBC membranes, agglutination occurs—a positive result. A positive result suggests the presence of autoantibodies to RBCs. A positive result due only to C3d suggests the presence of a cold autoantibody (usually IgM, which is not detected in the DAT since the Coombs serum is against IgG only). A positive test result does not always equate with hemolysis. Thus, results should always be correlated with clinical signs and symptoms.