Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (834 page)

   Proper specimen collection is required for reliable and accurate reporting of the CBC. Misleading results occur if the specimens contain clots, if the blood is not properly mixed, or in the presence of agglutinated RBCs. Specific pitfalls are described under each lineage.

COOMBS (ANTIGLOBULIN) TEST
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DIRECT COOMBS TEST (DAT)

   Definition

   The DAT is used to detect IgG antibodies or complement components on the RBC surface membrane by incubating Coombs (rabbit anti-human globulin [AHG]) reagent with the patient’s washed RBCs. The Coombs reagent can selectively bind IgG or C3 or be polyspecific and contain both anti-IgG and anti-C3.

   Use

   DAT is useful in diagnosing autoimmune hemolysis [see p. 377]), hemolytic diseases of the newborn [see p. 380]), drug-induced hemolysis, and hemolytic transfusion reactions [see p. 757]).

   The DAT is often used whenever hemolysis of red cells is suspected as being caused by autoantibodies. The assay determines if red cells have been coated in vivo with immunoglobulins, complement, or both.

   Interpretation

Positive DAT

   The DAT is positive whenever the patient’s red cells are coated with autoantibodies that developed against the patient’s own red cells.

   Hemolytic disease of the newborn (erythroblastosis fetalis).

   Warm autoimmune hemolytic anemias.

   Idiopathic.

   Evan syndrome (ITP and hemolytic anemia).