Which antigen to use in the detection of rheumatoid factors? Comparison of patients with rheumatoid arthritis and subjects with 'false positive' rheumatoid factor reactions.

In the search for differences between rheumatoid factors (RF) in patients with rheumatoid arthritis (RA) and in non-rheumatoid subjects, the reactivity of IgM- and IgA-class RF with rabbit IgG (rIgG) and Fc fragments of rabbit and human IgG (rFc, hFc) was studied by enzyme immunoassay. From a community-based cohort (n = 7124) representing the adult population of Finland, RA patients (n = 130), other subjects positive in the Waaler-Rose (WaRo, sensitized sheep cell agglutination) test (n = 137), and controls matched for age, sex and living area were selected for further study. In RA sera there was a good correlation between the results in the WaRo test and in the IgM-RF ELISA (rIgG, rFc and hFc). A considerable number of 'false positive' sera, though positive in the WaRo test and the rIgG-ELISA, were negative in the rFc-ELISA. Twenty-two per cent of the false positive sera reacted with either hFc or rFc, both types being equally common. IgA-RF reacted more frequently with hFc than with rFc in both the RA and the 'false positive' sera. In some sera, IgM- and IgA-RFs reacted differently with human and rabbit Fc, e.g. IgM-RF reacted only with human Fc, and IgA-RF reacted with both hFc and rFc, thus suggesting different regulation of their formation.