Validation of the PEG-IgG screening test for soluble immune complexes by longitudinal studies in experimental acute serum sickness

A wide variety of tests for the detection of circulating immune complexes (IC) has been proposed by different authors, but there is very little to no information concerning the performance of IC screening assays in samples known to contain in vivo-formed IC. The purpose of our investigation was to compare the behavior of a non-specific assay, the PEG-IgG screening test for IC, with an antigen-specific assay in serum samples sequentially obtained from rabbits to which we induced acute serum sickness. Five animals were used in the study; we were able to detect an increase of IC constituted by the heterologous antigen (human serum albumin) and corresponding antibodies in all, and in 4 animals the results of the PEG-IgG assay closely correlated with the results of the antigen-specific assay (rho values between 0.975 and 1.00). The 4 animals in which IC showed a definite peak by both assays developed proteinuria and IC deposits at the glomerular level, while the animal that failed to develop IC detectable by the PEG-IgG test remained normal throughout the study. These results demonstrate the ability of the PEG-IgG test to detect in vivo-formed IC and suggest that the IC detected by this test have pathogenic potential.