Serum C1q concentrations in rheumatic disorders. Early normalization during treatment of immunologically-mediated vasculitis.

Serum C1q concentrations were studied in 78 patients with definite and classical rheumatoid arthritis (RA). No correlation could be made either with disease activity, with erythrocyte sedimentation rate or with rheumatoid factor titers. In contrast to these findings in RA, however, serial determinations of this complement component were of value in predicting the response to therapy in four patients with immunologically-mediated vasculitis. Initially both C1q and C3 were depressed in two patients with SLE, one with cryoglobulinemic purpura and one with HbsAg-positive serum sickness, each with acute vasculitis. Sequential studies following the institution of treatment showed in each case that C1q returned to normal while C3 remained low. These observations suggest that analysis of serum C1q is preferable to C3 in these disorders when used to predict clinical change and the response to treatment.