The frequency of circulating immune complexes in rheumatoid arthritis and systemic lupus erythematosus

Summary Sera from patients with rheumatoid arthritis and systemic lupus erythematosus have been examined for the presence of complement-fixing immune complexes using three assays, (a) a fluid phase Clq binding assay, (b) a solid phase Clq binding assay and (c) the Raji cell assay.By simultaneously screening all the sera within each disease group we established that circulating immune complexes frequently occur but that there is a discordance between the assays, even between the two assays involving binding to Clq. Distinct clinical profiles emerged with the fluid phase Clq binding assay being most frequently positive in sera from patients with extra-articular rheumatoid arthritis. The solid phase Clq binding assay and the Raji cell assay were more frequently positive in sera from patients with systemic lupus erythematosus. The prevalence of circulating immune complexes and the comparative performance of the three assays in each disease is examined in detail.