Lipid interference in steroid radioimmunoassay.

To study lipid interference in steroid radioimmunoassays in which dextran-coated charcoal is used as the separating agent, we tested triolein and phosphatidylcholine as model hydrophobic and amphipathic lipids, respectively. Addition of either caused distortion of the standard curve to an extent that was inversely related to the polarity of the steroid molecule. Both lipids form a dispersion that entraps steroid molecules. When we increased the charcoal concentration, the effect of phosphatidylcholine addition was eliminated for assays of both polar and nonpolar steroids. In contrast, the effect from triacylglycerol was not corrected, particularly in assays of nonpolar steroids. We also studied mixtures of lipids mimicking the mixture of lipids extracted from plasma of normolipemic and hyperlipemic indviduals. The degree of lipemia that can be tolerated differs from assay to assay, and primarily varies directly with the polarity of the steroid being assayed.