Correlation between SF precipitate induced TNF-α production, IgG levels in SF precipitates, and RF (n = 15). Healthy PBMCs were stimulated with PEG precipitates from SF from 15 patients with RA. The stimulated cells were cultured for 20 hours at 37°C with 5% carbon dioxide. IgG levels in the SF precipitates correlated with (a) TNF-α production after SF PEG stimulation and (b) RF measured in serum. (c) There was also a nonsignificant positive correlation between RF and TNF-α induced by SF PEG precipitates. Statistical analyses were performed with nonparametric tests to diminish the effect of outliers. PBMC, peripheral blood mononuclear cell; PEG, polyethylene glycol; RA, rheumatoid arthritis; RF, rheumatoid factor; SF, synovial fluid; TNF, tumour necrosis factor.