Immunoglobulin class of rheumatoid factors detected by enzyme-linked immunosorbent assay

The immunoglobulin class of rheumatoid factor (RF) in sera and synovial fluids of patients with rheumatic diseases was measured by a solid-phase enzyme immunoassay. IgG-RF was frequently found in seronegative sera as well as in seropositive sera of rheumatic patients. Titers of IgG-RF were higher in the active phase than in the inactive phase of SLE and showed a correlation to the levels of hypocomplementemia and circulating immune complexes in SLE sera. Titers of IgA-RF were likewise higher in the active-phase of SLE with a correlation to titers of circulating immune complexes. IgA-RF was also found frequently in sera of patients with Sjögren's syndrome.