Paradoxical findings in direct antiglobulin test and classification of agglutinating autoantibodies using eluates and monospecific anti-human globulin sera

Background and ObjectivesAgglutinating autoantibodies are rarely the cause of autoimmune haemolytic anaemia (AIHA) of warm type. These antibodies can be difficult to classify, and serological testing may result in confusion. Here, we describe the occurrence of paradoxical results in direct antiglobulin test (DAT) and a simple technique for the characterization of such antibodies. Materials and MethodsSeven patients with AIHA were included in this study. Serological testing was performed using standard techniques. Classification of autoantibodies was performed by preincubation of the eluates from patients' red blood cells (RBCs) with monospecific anti-human globulin sera (mAHG). ResultsStrong autoagglutination of patients' RBCs was observed in six of seven cases, with the identification of panagglutinating serum antibodies in three patients. Initially, cold agglutinins with high thermal amplitude were suggested in four patients, and IgM warm autoantibodies were suggested in the remaining three patients. However, inhibition of the eluates revealed autoantibodies of IgA class in two patients, of IgM class in three other cases and of IgG class in two patients. The results of DAT were confusing due to paradoxical effects of mAHG and/or strong autoagglutination. ConclusionStrongly agglutinating autoantibodies can be a source of confusion and can be classified by inhibition experiments using eluates and monospecific antibodies.