J Clin Pathol 1987;40:107-1 11 Clinical value of ELISA assays for IgM and IgG rheumatoid factors

SUMMARY The clinical value of enzyme linked immunosorbent assays (ELISA) assays for IgM and IgG rheumatoid factors was assessed in a series of studies using rabbit IgG as antigen. The tests were reproducible with intra-assay coefficients of variation of 6 % and could be simply and rapidly performed. Normal ranges were established using 106 sera from healthy controls. In a cross sec-tional study of 208 rheumatoid patients these assays were compared with the Rose-Waaler and laser nephelometric assessments of rheumatoid factor. In some patients there were discrepancies between rheumatoid factor positivity determined by one method or another. IgM ELISA and Rose-Waaler titres showed a significant correlation (r = +0 58; p < 0-001), but there was a low correlation between IgM and IgG ELISA (r = +0-27; p < 0-001). There was no evidence to show that the measurement of IgM or IgG rheumatoid factor gave significantly more clinical information than traditional tests such as the Rose-Waaler or latex agglutination tests. Rheumatoid factors can be of any antibody class.' Established methods for their routine assessments, such as the latex and sheep red cell agglutination methods24 have disadvantages. They measure rheu