Circulating immune complexes in rheumatoid arthritis: a prospective study using five immunoassays.

We evaluated 257 patients with rheumatoid arthritis (RA) to determine the frequency of circulating immune complexes (CIC) in the serum, relationships between the presence of CIC, clinical indices of disease activity and other laboratory features, and relationships between changes in CIC levels and changes in disease activity. CIC were detected by fluid phase Clq binding activity, conglutinin binding activity, anti-C3 assay, staphylococci protein A binding assay and the precipitation of cryoglobulins. CIC in the serum were found to correlate with indices of disease activity, extraarticular features and the presence of rheumatoid factor. A change in Clq binding activity correlated with a parallel change in the joint count.