An enzyme immunoassay (EIA) was developed for measurement of complement-mediated solubilization of immune complexes. Preformed complexes consisting of horseradish peroxidase (Pe) and anti-Pe antibodies were incubated in diluted serum. After centrifugation the soluble complexes in the supernatant were quantitated by adding peroxidase substrate followed by measurement of absorbance. Kinetic analysis of revealed significant difference between normal and EDTA-treated serum similarly to a standard assay utilising immune complexes containing radiolabelled bovine serum albumin. The difference was most pronounced after incubation of immune complexes with serum for 40 minutes. Immune complex solubilization measured by EIA was reduced in sera from 10 patients with active systemic lupus erythematosus (SLE). In serially followed patients, flares of SLE were preceeded by reduction of solubilization capacity. The EIA is a simple method for determining serum capacity to solubilize preformed immune complexes and might be considered as a routine test for assessment of complement function in diseases such as SLE.
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