Detection of circulating immune complexes in human sera by simplified assays with polyethylene glycol

The search for circulating immune complexes by precipitation tests using polyethylene glycol (PEG) was performed on a series of normal and pathological sera. Various factors affecting PEG precipitation were studied. Immunoglobulins and complement factors percipitated by PEG (3.5%) were quantified and their significance was discussed in relation to serum levels. The PEG test was compared to labeled C1q binding test with a fairly good correlation. The direct evaluation of the amount of C4 precipitated with IgG by 3% PEG (C4 test) provided a simpler routine assay than the C1q binding test for detecting complement-fixing immune complexes. The direct PEG test and the C4 tests gave positive results in patients with diseases generally presumed to be associated with immune complexes including systemic lupus erythematosus, acute glomerulonephritis, bacterial sub-acute endocarditis and chronic acitve hepatitis. The demonstration of HBs antigen and antibody after acid dissociation of PEG precipitates from hepatitis B seronegative sera illustrated the fact that PEG does precipitate and thus concentrates circulating immune complexes.