Eluate testing following microscopically positive direct antiglobulin tests with anti-IgG.

The direct antiglobulin test (DAT) demonstrates the presence of immunoglobulin (eg, IgG) or complement on the surface of red blood cells (RBCs). Immunoglobulin can be removed from RBCs by elution. The liquid end-product of elution procedures, the eluate, can be evaluated by antibody identification procedures. Antibody identification studies following acid/EDTA elution and DATs performed in our immunohematology laboratory during 2005 were evaluated to determine the usefulness of eluate testing following a microscopically positive IgG DAT. In total, 310 eluates were prepared during the year 2005; 146 of these eluates were derived from RBC samples with a microscopically positive DAT. The remaining eluates were derived from RBC samples with macroscopically positive IgG DATs (85 were weakly +; 32 were 1+; 40 were 2+; and 7 were 3+). Data were collected for the number and types of new antibodies (warm autoantibodies or alloantibodies) that were identified as a consequence of eluate testing. The incidence of new alloantibodies in the microscopically positive group (0.7%) was significantly lower than the combined incidence in the macroscopically positive groups (5.5%, p = 0.02). Based on these results, the authors conclude that performing antibody identification procedures on acid/EDTA eluates derived from RBC samples with microscopically positive IgG DATs has limited utility.