Measurement of immune complexes with the liquid phase C1q binding assay: ten years experience in a routine diagnostic...

We describe our 10 years experience in assaying over 15,000 clinical specimens for immune complexes (IC) using the C1q binding assay. Normal ranges were initially established using a large panel of blood donor sera and precision of the assay was optimized by inclusion of heat aggregated IgG (HAGG) as standards. Nevertheless some variability was observed due to variation in C1q binding from batch to batch and with aging of this reagent. In an empirically selected 2 year period involving over 3,000 clinical specimens, 25% had elevated concentrations of IC. Of these the majority were from patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), other connective tissue disorders, infective endocarditis (IE), diffuse interstitial lung disease (DILD) and vasculitis (VASC). In RA, IE and VASC, significant correlations were observed between concentrations of IC and rheumatoid factor (RF) and the addition of a purified monoclonal RF to normal serum caused increased C1q binding. Longitudinal studies in RA and IE demonstrated a striking decline in IC in response to effective treatment. We conclude that the measurement of IC provides little additional useful diagnostic information in those diseases associated with high levels of RF but appears more useful in disorders such as SLE, IE and DILD in which RF is absent or present in low concentration. Sequential monitoring of IC in RA and IE reflects response to treatment.