A standardized method for quantitating the complement-mediated immune complex solubilizing capacity of human serum.

A standardized radioassay for measuring the complement-mediated immune complex solubilizing capacity (CMSC) and the initial kinetics of the solubilization (IKS) reaction is described. The total complement (C)-mediated solubilizing capacity was determined after incubation of diluted serum and 125I-BSA-anti-BSA. Percentage C-mediated solubilization (CMS) was measured after centrifugation by determining the distribution of radioactivity. The dependency of CMSC upon factors such as serum dilution and buffer system used, amount of IC added to serum, serum storage conditions and centrifugation conditions was investigated in order to optimize the assay. The CVt of the standardized assay was 0.10-0.17 depending upon the CMSC level measured. Treatment which inactivates C factors (heating), interferes with C activation (EDTA) or activates and consumes C components (zymosan) markedly reduces the CMSC. Preliminary investigation of pathological sera showed that both IKS and CMSC were clearly reduced in SLE sera. By contrast, rheumatoid arthritis sera exhibited normal IKS and only marginal reduction in CMSC.