Quantitative solid phase fluorescence immunoassay of rheumatoid factor and C-reactive protein in active rheumatoid arthritis

A quantitative, semiautomated, solid-phase fluorescence immunoassay (FIA) has been developed for measuring rheumatoid factor (RF) and C-reactive protein (CRP). The correlation of the FIA measurement of RF and CRP with standard measurements in rheumatoid arthritis (RA) patients is not known. To determine the correlation of FIA with standard assay methods, RF and CRP levels were measured by both methods in 151 patients with active RA. RF levels measured by FIA correlated very closely with charcoal agglutination method (r2 = 0.890, P less than 0.0001). CRP levels by FIA correlated very closely with CRP levels by nephelometric method (r2 = 0.886, P less than 0.0001) and Westergren erythrocyte sedimentation rates (ESR) (r2 = 0.356, P less than 0.0001). A weak statistical correlation of RF, CRP, and ESR with some clinical variables of RA disease activity was demonstrated. Measurement of RF and CRP by FIA is similar to standard methods and offers no specific advantages in evaluating RA patients at a single evaluation.