Recovery of immune complexes from serum-based samples is dependent on the concentration of reactants:...

We examined heat-treated and untreated samples for immune complexes (IC) with a nephelometric type Clq binding assay. Curves of absorbance vs. concentration derived from aggregated gamma-globulin in serum samples showed curvilinear behavior, while curves obtained from aqueous samples were highly linear. Curves from heat-treated sera bent upward, while those from unheated sera tended to level off, causing the curves to cross. Heated sera showed greater recovery of IC in the high range and unheated sera greater recovery of IC in the low. Recovery from serum-based samples varied as the concentrations of both Clq and IC in the reaction mixture changed when aqueous curves were used as calibrators. This behavior in serum-based samples seems to be caused by endogenous binders of IC, and may be an important reason for inter-laboratory variation in results from similar samples.