The measurement of 24-hours free urinary cortisol is considered by many authors as the best screening test for Cushing's syndrome. However, the published results are not comparable due to methodological differences between the various available techniques. These differences are related mainly to the preparative process employed, and in general there is a direct relation between the complexity of the process and the degree of specificity achieved. In this publication we describe the adaptation for free urinary cortisol measurement of a routine assay, developed originally for the measurement of serum cortisol. The specificity of the antiserum employed, F-79-1, was studied against the major urinary cortisol metabolites, and the results comparable to the best antisera described in the literature. The preparative method utilized includes dichloromethane extraction followed by a Sep-Pak Diol column chromatography. The mean recovery of the preparative process was 63.3%; the sensitivity of the assay is in the order of 1.6 mug/L and the intra and inter-assay CVs in the order of 7.2 and 15.6%, respectively. The results obtained in 40 samples from normal adults ranged betwen 21.9 and 74.1 mug/24hs (mean 47.8); the comparison with a similar method from an international reference laboratory showed a high correlation (R= 0.9286, n= 74, p<0.001). In four patients with Cushing's disease the values were significantly above the normal range. The method described, employing our own antiserum and a simple preparative process, will allow a broader use of the test, and a better clinical evaluation of its practical value.
Free urinary cortisol; Radioimmunoassay; Anti-cortisol antiserum especificity; Cushing's disease
