Direct Antiglobulin Test
The direct antiglobulin test (or direct Coombs' test) detects immunoglobulins (antibodies) on the surface of red blood cells (RBCs). These immunoglobulins coat RBCs when they become sensitized to an antigen, such as the Rh factor.
In this test, antiglobulin (Coombs') serum added to saline-washed RBCs results in agglutination if immunoglobulins or complement is present. This test is "direct" because it requires only one step- the addition of Coombs' serum to washed cells.
Procedure and posttest care
A negative test, in which neither antibodies nor complement appears on the RBCs, is normal.
A positive test on umbilical cord blood indicates that maternal antibodies have crossed the placenta and coated fetal RBCs, causing HDN. Transfusion of compatible blood lacking the antigens to these maternal antibodies may be necessary to prevent anemia.
In other patients, a positive test result may indicate hemolytic anemia and help differentiate between autoimmune and secondary hemolytic anemia, which can be drug-induced or associated with an underlying disease. A positive test can also indicate sepsis.
A weakly positive test may suggest a transfusion reaction in which the patient's antibodies react with transfused RBCs containing the corresponding antigen.
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