Which results on a direct antiglobulin test (DAT) are characteristic of autoimmune hemolytic anemia?

Updated: Jun 25, 2020
  • Author: Julie Katz Karp, MD; Chief Editor: Jun Teruya, MD, DSc, FCAP  more...
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Conditions in this category include warm autoimmune hemolytic anemia, cold agglutinin syndrome, mixed- or combined-type autoimmune hemolytic anemia, and paroxysmal cold hemoglobinuria. [4]

Warm autoimmune hemolytic anemia is the most common type of autoimmune hemolytic anemia and is generally associated with warm-reactive IgG autoantibodies. Three patterns of reactivity may be found on a direct antiglobulin test: IgG alone, complement alone, or both.

Cold agglutinin syndrome is associated with cold-reactive autoantibodies, most frequently immunoglobulin M (IgM). In areas of low temperature in the peripheral circulation, IgM binds to the RBCs and causes complement to also adhere to the RBCs. As the RBCs circulate to areas of higher temperature, the IgM may dissociate, but the complement remains. Additionally, the standard direct antiglobulin test will not detect IgM coating of RBCs. As such, complement alone is commonly detected on the direct antiglobulin test.

In a patient with mixed-type autoimmune hemolytic anemia, the direct antiglobulin test commonly detects both IgG and C3.

Paroxysmal cold hemoglobinuria is caused by a biphasic hemolysin, known as the Donath-Landsteiner hemolysin. This autoantibody is an IgG complement-fixing antibody that reacts with RBCs in the colder areas of the circulation. The IgG causes C3 to bind irreversibly to RBCs; subsequently, the IgG dissociates from RBCs in warmer parts of the body. Thus, the direct antiglobulin test is typically positive only for complement.

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