ELISA assays for IgM and IgG rheumatoid factors: Their clinical correlations during therapy with slow-acting anti-rheumatic drugs

A longitudinal study of changes in rheumatoid factor levels and their correlations with other clinical and laboratory assessment of disease activity followed 45 rheumatoid arthritis patients during 12 months continuous therapy with gold and penicillamine. Rheumatoid factors were measured by the titred latex assay and by ELISA assays for IgM and IgG rheumatoid factor. The IgM ELISA assay correlated strongly with the titred latex assay and weakly with the IgG rheumatoid factor assay at 0, 6 and 12 months. All showed significant falls with treatment, and ELISA assays were not more sensitive indicators than the latex assay. Correlations of initial rheumatoid factor levels by all 3 assays to 34 other clinical and laboratory assessments showed that only a small number of weak correlations existed; there were more at 6 and 12 months, but still not many. Changes in rheumatoid factor levels over 0–6 months showed no significant correlation to changes in other variables. There were no more correlations with ELISA assays than with titred latex rheumatoid factor. We conclude that ELISA rheumatoid factor assays have no clinically relevant advantage over the titred latex assay when following rheumatoid patients treated with gold or penicillamine.