Selective determination of urinary free cortisol by liquid chromatography after solid-state extraction.

We have developed a selective and precise high-performance liquid chromatographic method for urinary free cortisol with an improved and efficient sample clean-up using C18 Sep-Pak cartridges. The urine sample (2 ml), with 11-deoxycortisol as internal standard, is applied to the Sep-Pak, which is then sequentially washed with acetone-water (1:4, v/v), water and hexane. Cortisol is eluted with diethyl ether, evaporated to dryness and redissolved in 2 ml of water. The wash cycle is repeated once using the same Sep-Pak cartridge. This double extraction greatly improves sample clean-up and allows modification of the mobile phase (tetrahydrofuran-methanol-water) so that cortisol is rapidly eluted as a single well resolved peak at 13 min. Chromatography is performed isocratically on a reversed-phase column with detection at 254 nm. Detection limits for urinary free cortisol by this procedure were two or three times lower than those obtained with two commercial radioimmunoassay kits. The chromatographic method was used successfully in the diagnosis of patients with hypercortisolism and Cushing's syndrome.