Serum interleukin 6 levels in rheumatoid arthritis: correlations with clinical and laboratory indices of disease activity

In rheumatoid synovium interleukin 6 (IL-6) is the most abundantly expressed cytokine. Increased serum levels have been previously reported in patients with rheumatoid arthritis (RA). In this study serum IL-6 levels were measured in a well defined cohort using a bioassay (B9 cells) and levels were correlated with conventional clinical and laboratory indices of disease activity. Levels were significantly higher in serum from patients with RA (median 55 IU/ml; interquartile range 28-139) compared with serum from disease (median 7 IU/ml; 1-23) and normal controls (median 10 IU/ml; 7-12). No difference was observed between men and women. Levels did not correlate with disease duration. Significant associations were observed between IL-6 and C reactive protein and between the Ritchie articular index and duration of morning stiffness. No other correlations were observed. The value of these findings in the monitoring of RA and as an indicator of response to second line treatment needs to be established.