Absorbance nephelometry of immune complexes by reaction with anti-IgG after treatment with polyethylene glycol.

We describe absorbance nephelometry of immunoglobulin G (IgG) in immune complexes (IC) with use of anti-IgG after precipitating the IgG from serum by three treatments with polyethylene glycol, 25 g/L final concentration. The three-step procedure removed all of the monomeric IgG, eliminating interference with the IC assay. Analytical recovery of IC was nearly 100%. Both the standard curve and results for control sera showed good day-to-day reproducibility, with CVs of less than 8%. Eighteen of 20 patients with diseases involving IC showed distinctly increased concentrations of IC; no increases were seen in 15 normal persons. Measurement of IC in 44 samples by this assay and a C1q assay gave a Spearman rank correlation coefficient of 0.76. The assay appears suited to routine use with manual double-beam spectrophotometers and stable nephelometers. The procedure is inexpensive, straightforward, and easy to perform.