Absorbance nephelometry of C1q-precipitable immune complexes: method comparisons and clinical correlations

Using a double-beam spectrophotometer, we investigated the clinical utility of a nephelometric method for assaying immune complexes. The complexes were concentrated from serum by precipitation with polyethylene glycol (PEG) and assayed by reaction with C1q. Testing of more than 100 sera showed a Spearman's rank correlation (p) between the present assay and the C1q-binding assay of 0.57, and 0.39 between the Raji cell assay and the present assay. Clinical sensitivity of the methods was not statistically different (p less than or equal to 0.5). Twenty-four of 30 patients with symptoms of disease showed increased concentrations of immune complexes by the present assay; only one of 38 normal individuals showed an increase. In a longitudinal study, we found that the concentrations of immune complexes paralleled clinical changes, indicating good clinical utility. The use of this assay with single-beam analyzers is limited because of the poor aqueous solubility of the PEG precipitate. Ongoing investigations designed to circumvent this problem are described.