Detection of immune complexes in systemic lupus erythematosus with the C1q solid phase assay: correlation with nDNA antibodies and hypocomplementemia.

Three immune complex assays, the monoclonal rheumatoid factor inhibition (mRF), the C1q solid phase (C1q-SP), and the C1q fluid phase binding (C1q-BA) assays, were compared with native DNA antibody (nDNA Ab) titers and serum hemolytic complement (CH50) levels in serial analyses of patients with systemic lupus erythematosus (SLE). No correlation was evident among the immune complex assays. A positive correlation was observed between the C1q-SP and nDNA Ab assays, and a negative correlation was observed between the C1q-SP and CH50 assays. Evidence is presented that these correlations are related to the presence of complement-fixing nDNA Ab. The C1q-SP may be useful in delineating the basis for the unexplained association of nDNA Ab and hypocomplementemia.