A Comprehensive Diagnostic Algorithm for Direct Antiglobulin Test-Negative Autoimmune Hemolytic Anemia Reveals the Relative Ratio of Three Mechanisms in a Single Laboratory.

BACKGROUND

Direct antiglobulin test (DAT)-negative warm autoimmune hemolytic anemia (AIHA) is mainly caused by three mechanisms: red blood cell (RBC)-bound immunoglobulin (Ig)G below the detection limit of routine DAT; RBC-bound IgA or IgM; or low-affinity autoantibodies. Although most cases of DAT-negative AIHA are thought to be caused by RBC-bound IgG, and combinatory serological analyses are recommended, the relative ratios of each mechanism have not been clarified.

METHODS

Two groups of patients with undiagnosed hemolytic anemia and negative conventional tube method-DAT (TM-DAT) were investigated using anti-IgA and anti-IgM sera, or column agglutination method-DAT (CM-DAT), respectively, in addition to radioimmunological quantitation of RBC-bound IgG.

RESULTS

Three of 73 patients with DAT-negative AIHA showed positive RBC-bound IgA and normal amounts of RBC-bound IgG. Another group of 3 patients were RBC-bound IgM-positive, but only one of these showed normal amounts of RBC-bound IgG. In another group of patients with DAT-negative AIHA, 4 of the 20 showed positive CM-DAT and negative CM-DAT after washing RBCs. Three of these patients had normal amounts of RBC-bound IgG. Five patients with positive CM-DAT both before and after washing RBCs had high amounts of RBC-bound IgG.

CONCLUSION

Relative ratios of patients with DAT-negative AIHA resulting from RBC-bound IgG, RBC-bound IgA, RBC-bound IgM, and low-affinity IgG were estimated as 80, 4, 1 and 15%, respectively. A new classification and diagnostic algorithm for DAT-negative AIHA were proposed.