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The direct antiglobulin test is used to detect the presence of immunoglobulin and/or complement on the surface of red blood cells. This test is helpful in the work-up of autoimmune hemolytic anemia, drug induced hemolytic anemia, hemolytic disease of the newborn, and hemolytic transfusion reactions. A positive direct antiglobulin test indicates that antibody and/or complement has sensitized the patient's RBCs in vivo. The strength of the direct antiglobulin test does not predict the biological activity of antibodies. For instance, some patients with a strongly positive direct antiglobulin test have little hemolysis, while other patients with weakly positive or negative direct antiglobulin test hemolyze extensively. Also, the strength of the direct antiglobulin test often does not change following treatment, even though the clinical condition greatly improves. Approximately 1 in 9000 healthy persons has a positive direct antiglobulin test with no evidence of hemolysis. Autoimmune hemolytic anemia is classified according to the serological characteristics of the RBC autoantibody. In warm autoimmune hemolytic anemia, RBCs may be coated with IgG, IgG and complement, or complement alone. In contrast, cold autoimmune hemolytic anemia is caused by complement-fixing IgM antibodies that react more strongly in the cold than at higher temperatures. In these cases, the direct antiglobulin test detects only complement. Results are reported as negative or positive. Reference value is a negative reaction. Specimen requirement is one 5 mL lavender top (EDTA) tube of blood. |
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