IgM rheumatoid factor estimation by ELISA in seronegative rheumatoid arthritis before and after IgM fractionation: does seronegative RA exist?

We looked for the presence of IgM rheumatoid factor (RF) using an enzyme-linked immunosorbent assay (ELISA) in 25 patients with active seronegative rheumatoid arthritis. In unfractionated sera, 12 patients (48%) were positive for IgM RF (classical), but after IgM fractionation of 23 samples (2 samples were not available) using high performance liquid chromatography for fractionating IgM, 12 patients were positive for RF by ELISA indicating the presence of hidden RF. Finally, three patients were labelled as truly seronegative for IgM RF. Classical IgM RF as detected by ELISA correlated significantly with erosive disease. Hidden RF did not correlate with disease activity or severity in this cross-sectional study, and though its presence was associated with shorter disease duration this did not reach statistical significance.