Effects of rheumatoid factor isotypes on disease activity and severity in patients with rheumatoid arthritis: a comparative study

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Abstract
The value of rheumatoid factor (RF) isotypes for assessing rheumatoid arthritis (RA) remains debatable. In this study, we have examined the relationships between RF isotypes and disease activity and severity in RA patients. Sixty-two patients with RA, 48 women and 14 men, were studied. RF was measured by nephelometry (RF–N) and IgG–, IgA–, and IgM–RF isotypes were measured using enzyme-linked immunosorbent assay. Serum C-reactive protein and erythrocyte sedimentation rate were also determined. The patients were classified according to disease activity, joint damage, functional status, and presence of pulmonary involvement, rheumatoid nodule, and secondary Sjögren’s syndrome. Although the patients with active disease had significantly higher IgA–RF and IgM–RF levels compared to inactive patients, IgA–RF and IgM–RF were not found to be independently associated with disease activity in multivariate analysis. In patients with severe joint damage, IgA–RF and RF–N were significantly higher than those of the other patients. Multiple regression analysis showed that IgA–RF was the unique variable independently associated to severe joint damage. The patients with class III and IV functional index had significantly higher IgM–RF, IgA–RF, and RF–N levels compared to the patients with class I and II functional index; however, RFs were not significantly associated with functional status in multivariate analysis. IgA–RF and IgM–RF were significantly associated with pulmonary involvement and rheumatoid nodule, respectively. No significant associations were found between RF isotypes and secondary Sjögren’s syndrome. Our results suggest that the clinical usefulness of IgA and IgM isotypes is better than RF–N. Elevated IgA–RF may be a marker of erosive disease. The usefulness of RF isotypes for monitoring disease activity or functional status appears to be limited.