Technical falsely positive rheumatoid factor by ELISA in sera with elevated IgM levels.

One half of sera with monoclonally or polyclonally raised IgM levels showed substantial IgM adsorption on to polystyrene plates coated with tetanus toxoid-human antitoxin complexes, plain tetanus toxoid and human serum albumin. In three latex-negative sera out of 56 sera with monoclonal IgM components this adsorption exceeded the rheumatoid factor binding from rheumatoid sera on to plates coated with tetanus toxoid-human antitoxin complex. The nonspecificity of this IgM adsorption was further substantiated by the lack of inhibition with heat-aggregated human immunoglobulin. The falsely positive rheumatoid factor phenomenon was largely restricted to sera with raised IgM levels.