Interferences in the radioimmunological determination of urinary free cortisol

The magnitude of interferences arising in the radioimmunological assessment of urinary free cortisol is studied (a) by comparing cortisol immunoreactivities from crude urine, after organic solvent extraction of different selectivity and after additional chromatography by high pressure liquid chromatography (HPLC) and (b) by evaluation of the profile of immunoreactivity resulting from the fractions eluted by HPLC. Three antisera from different sources have been investigated. Values of cortisol-immunoreactivity in crude urine were about six times and values of a simple dichloromethane extract about three times higher than values obtained after HPLC. The main part of the interfering compounds arising in organic extracts have a polarity similar to cortisol, which cannot be easily eliminated by simple solvent extraction procedures. Specific estimation of urinary cortisol by radioimmunoassay requires a preceding chromatographic technique of high efficiency, such as HPLC, which represents an adequate tool for the routine laboratory.