Evaluation of anti-C1q capture assay for detecting circulation immune complexes and comparison with polyethylene glycol-immunoglobulin G, C1q-binding, and Raji cell methods

Abstract

An anti-C1q capture method kit (C1q-immunoglobulin G[IgG]) (Ortho Diagnostics, Inc., Raritan, N.J.) for measuring circulating immune complexes (CIC) was evaluated. The kit showed poor diagnostic sensitivity (P<0.005) for identifying CIC in patients with systemic lupus, rheumatoid arthritis, and bacterial endocarditis, as compared with polyethylene glycol-IgG and Raji cell tests (12, 24, and 24 positive, respectively, of 31 patients). Of the patients who were positive with the C1q-IgG test, 25% showed discrepancies when their results were compared with the polyethylene glycol-IgG and C1q-binding test results. Gel filtration chromatography of two of these discrepant sera showed the only peak of C1q-IgG activity to be associated with monomeric IgG (molecular weight, <200,00). We conclude that the kit method may be measuring substances other than CIC in some sera, because molecules of C1q attached to IgG should exhibit a molecular weight of greater than 500,000.