Attempts to support an immune etiology in 800 patients with direct antiglobulin test–negative hemolytic anemia

Clinical and hematologic evidence of warm autoimmune hemolytic anemia (AIHA) is present in some patients whose direct antiglobulin test (DAT) is negative. The most common causes for AIHA associated with a negative DAT are RBC-bound IgG below the sensitivity threshold of the DAT, RBC-bound IgA and IgM not detectable by routine reagents, and low-affinity IgG that dissociates during the testing process. Samples submitted from 800 patients with hemolytic anemia and a negative DAT were tested by an antiglobulin sera (AGS) panel of anti-IgG, anti-C3, anti-IgM, and anti-IgA by a routine DAT. Additional tests included a direct Polybrene test to detect small amounts of RBC-bound IgG, a cold-wash technique to detect low-affinity IgG, and a DAT by gel test with anti-IgG. A positive result was obtained with at least one method for 431 (54%) of 800 specimens tested. The AGS panel was positive for 400 (50%) of samples, with IgG or C3 or both accounting for reactivity in 48 percent. IgA alone was found on 2 percent of samples; IgM was never found alone. Low-affinity IgG was found on 37 (5%) samples. The direct Polybrene test was the only positive test for 15 (2%) samples. The gel anti-IgG test was never the only positive test. Clinical correlations for these data were not available; however, previously published correlations suggest a positive predictive value for tests that extend routine DAT methods in patients with DAT-negative AIHA. Immunohematology2010;26:156–60.