IMPROVED METHOD FOR MEASURING C1(R)OVER-BAR-C1(S)OVER-BAR-(C1 INH)(2) COMPLEXES BY AN ENZYME-LINKED-IMMUNOSORBENT-ASSAY

Measurement of C1(r) over bar-C1(s) over bar-(C1 inh)(2) complexes in serum or plasma by enzyme-linked immunosorbent assay (ELISA) has been proposed as a relatively convenient and sensitive means for assessing C1 activation. However, interference by unactivated C1q (r-s)(2) at low serum or plasma dilutions has resulted in estimates that vary widely with the degree of serum or plasma dilution. Precipitating the interfering C1q (r-s)(2) with 6% polyethylene glycol has been proposed to resolve this problem, but here it is shown that this procedure also precipitates or coprecipitates some of the C1(r) over bar-C1(s) over bar-(C1 inh)(2) complexes. Satisfactory results have been achieved without PEG precipitation by testing high plasma dilutions under conditions where there is a sufficient excess of anti-C1s coating the microtitration plate wells that removal of C1q (r-s)(2) is not necessary. Optimizing conditions for quantitating these complexes at high dilution have been investigated. The mean normal EDTA plasma C1(r) over bar-C1(s) over bar-(C1 inh)(2) complex measurement was 36.6+/-7.0 (S.D.) ELISA units with a 95% confidence interval of 19.5-47.6u. Besides providing a sensitive assay for C1 activation, measuring C1(r) over bar-C1(s) over bar-(C1 inh)(2) complexes may help to clarify the pathophysiologic mechanisms resulting from C1 inh deficiency under various conditions. (C) 1995 Wiley-Liss, Inc.