A simple light-scattering method for detecting soluble immune complexes in human serum.

A simple assay based on molecular light scattering was developed to detect soluble immune complexes. In this assay the percentage of relative light scattering (%RLS) caused by complexes in buffer containing polyethylene glycol 6000 (PEG) was measured with a laser nephelometer. In low concentrations of PEG (3%-5%), IgG-anti-IgG complexes formed in antigen excess scattered light more effectively than those formed in antibody excess or equivalence. Since normal serum proteins caused minimal %RLS at 3%-5% PEG, we expanded this assay to screen patient sera. For each test, the serum was used at a 1:20 dilution. The results were expressed by the defined parameter delta %RLS = (sigma %RLS 3%-5% PEG - sigma %RLS 0%-2% PRG). For 58 normal sera delta %RLS = 15.4% +/- 11.3%, while for 12 sera from patients who had systemic lupus erythematosus and hypocomplementemia, delta %RLS = 40.5% +/- 20.0% (P < 0.001). In a comparative study of 18 sera positive for immune complexes in the Clq deviation assay, 12 were positive by %RLS.