What are the limitations of a direct antiglobulin test (DAT)?

Updated: Jun 25, 2020
  • Author: Julie Katz Karp, MD; Chief Editor: Jun Teruya, MD, DSc, FCAP  more...
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Answer

Answer

Polyspecific and anti-IgG reagents are very sensitive and can detect as few as 200-500 molecules of IgG per RBC. However, patients may experience autoimmune hemolytic anemia when IgG coating is below this level. As such, the direct antiglobulin test (DAT) result be negative despite the presence of IgG-coated RBCs. [5] This entity is sometimes referred to as direct antiglobulin test–negative autoimmune hemolytic anemia. [7] Therefore, even if the result is negative, elution may be performed if immune hemolysis is suspected.

If all of the test tubes and the negative control demonstrate agglutination, spontaneous agglutination is likely occurring. This may be caused by IgM coating. Spontaneous agglutination must be resolved before further testing. [4, 8]

False-positive results may be caused by the following:

  • Overcentrifugation or contaminated reagents [8]

  • Insufficient washing of the patient's RBCs [8]

  • If the test tubes were left to stand following centrifugation or if the RBCs were left in suspension for an extended period before testing [8]

Hemolytic transfusion reactions may result in a positive direct antiglobulin test result if sensitized RBCs have not been destroyed or a negative result if hemolysis and rapid clearance have occurred. [4]


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