Measurement of circulating Clq precipitable immune complexes by a nephelometric type assay after polyethylene glycol extraction in model system.

Clq precipitins were assayed in a model system by absorption nephelometry. Polyethylene glycol was used to extract immune complexes so that sufficient sensitivity with minimal interference could be achieved. Using a double beam spectrophotometer, we found that the reaction of immune complexes with Clq occurred rapidly and plateaued between 5-20 minutes. The optimal concentration of sodium in the reaction mixture was between 0.155-0.165 M, while chloride and EDTA did not affect the reaction significantly. Standard curves generated from heated aggregated gamma globulin showed a high degree of linearity, between 12.5-400 mg/L. Need for conductivity meters, radioisotopes, and other special equipment was eliminated because highly purified Clq is not required for nephelometric assay, and all necessary parameters can be monitored by flame photometry and electrophoresis, which are available in most clinical laboratories. Day to day reproducibility showed CVs of 5.4, 9.5, and 11.4% at control levels of 210, 160, and 70 mg/ L of immune complexes, respectively. Twelve patients with elevated levels of immune complexes by other methods were differentiated from normals by the present method. Nephelometry may offer a simplified approach for testing immune complexes and may become more widespread in clinical laboratories.