Meaning and Uses of theDirect Antiglobulin Test Marilyn Telen, MD Duke University
Direct Antiglobulin Test DAT Direct Coombs test Coombs test
Direct Antiglobulin Test • Test for in-vivo coating of patient’s red blood cells with anti-IgG and/or complement • Washed patient’s red blood cells + polyspecific anti-human globulin (AHG: anti-IgG, anti-C3/C3d) • If positive, test red cells with 2 monospecific antibodies (anti-IgG and anti-C3) • If IgG+ may remove (elute) antibody and test for specificity
Autologous Control • Positive autocontrol on panel can mean: • Alloantibody coating donor (previously transfused) red cells • Autoantibody
Direct Antiglobulin Test • Physician request – usually because immune-mediated hemolysis is suspected. • Positive DAT could mean: • Warm autoantibody • Cold autoantibody • Transfusion reaction • Hemolytic disease of the newborn • Drug induced antibody • Drug adsorption - penicillin • Immune complex • Membrane modification • Induced autoimmunity
DAT in Autoimmune Hemolytic Anemia (AIHA) • Warm AIHA – 80% of all AIHA • 99% have + DAT • 20% have IgG only • 67% have IgG + C3 • 13% have C3 only • Cold AIHA – 18% of AIHA • C3 only (antibody is IgM) • Paroxysmal Cold Hemoglobinuria – 2% of adult AIHA • C3 only or negative DAT
Direct Antiglobulin Test • Transfusion Reaction work-up • DAT to detect antibody coating only donor cells in post sample • Immediate hemolysis • Prior work up missed detectable antibody • Delayed hemolysis • Antibody was below detectable levels on pretransfusion testing
Direct Antiglobulin Test • Done with newborn type and screen • Hemolytic disease of the newborn • Maternal IgG antibody coating infant’s red cell • Anti-A, -B, -A,B • Alloantibody • Usually not due to Rh Immune Globulin
Direct Antiglobulin Test A positive DAT does not necessarily mean hemolysis is really occurring. A large number of hospitalized patients (8-30%) have a positive DAT but no evidence of hemolysis. Frequency of + DAT increases with concentration of immunoglobulin, and thus presence of inflammation. About 1% of autoimmune hemolytic anemia will have a negative DAT.
Direct Antiglobulin Test Useful in confirming immune-mediated hemolysis from a variety of causes. In combination with other tests, may help define cause of hemolysis. NOT indicative of hemolysis by itself.