Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (836 page)

   Finding of a positive DAT indicates the presence of red cell autoantibodies, alloantibodies following transfusions, or of coating of red cells with excess immunoglobulins. It requires additional workup to elucidate the etiology of the immunoglobulins by performing tests for antibody specificity: cold agglutinins (see p. 377 under hemolytic anemias), Donath-Landsteiner antibody (see p. 379), and also serum protein electrophoresis or immunofixation when a plasmacytic disease (see p. 432) is suspected. The administration of certain drugs (α-methyldopa, IV penicillin, or procainamide) and recent transfusions must also be excluded.

   False-positive results may occur in plasma cell myeloma and lymphoplasmacytic lymphoma.

   1:10,000 normal blood donors have a positive DAT.

   A negative DAT can rarely be seen in patients with autoimmune hemolytic anemia if only a small amount of IgG is bound to the RBC membrane.

   A negative DAT does not rule out hemolysis. For instance, DAT is negative in some cases of drug-induced hemolytic anemias, hemoglobinopathies, hereditary spherocytosis, and other hereditary hemolytic anemias.

INDIRECT COOMBS TEST (IAT)

   Definition

   The IAT uses the patient’s serum (or plasma), which is incubated with reagent or donor RBCs. Subsequently, this mixture is washed to remove unbound globulins and then incubated with Coombs reagent. Agglutination is seen if the patient’s serum contains antibodies against the RBCs.

   In about 80% of patients with autoimmune hemolytic anemia, the autoantibodies are also present in serum.

   Alloantibodies to RBC antigens induced by previous blood transfusions or fetal–maternal incompatibility are also detected by this assay. These alloantibodies are usually present only in serum because they do not bind to the patient’s RBCs, and the DAT is negative in these cases.

   Use

   The utility of the IAT in blood banking stems from its great sensitivity in detecting various IgG antibodies in a patient’s serum. It is used to detect the presence of alloantibodies directed against non-ABO blood group antigens.

   Antibody screening and cross-matching prior to blood transfusions.