C1q-latex assay for immune complexes. Complexes that react with both C1q and monoclonal rheumatoid factor in lupus...

A solid phase radioassay for measurement of ICs in biological fluids is described in which ICs present in test sample bind to C1q immobilized on latex particles and bound complexes are quantitated by reaction with radioiodinated mRF. The radioassay can reproducibly measure 10 ng of aggregated human IgG in serum and differentiate soluble complexes from IC-like materials that precipitate with centrifugation or low temperature or stick to test tube walls. Reagents used in the assay, including C1q-L, can be stored for extended periods of time before use. One hundred four of 171 sera from patients with SLE and 8 of 50 sera from patients with LC, assayed by this method, contained elevated levels of ICs relative to controls . IC levels determined by this method correlated with IC data generated by 125I-C1q-PEG precipitation. Raji cell radioimmune assay, and solid-phase conglutinin assay, in some cases but not other.