Radioimmunoassay of urinary cortisol: comparison of the procedures with and without previous cortisol extraction.

Although only less than 1% of the daily secreted cortisol is excreted unchanged into urine, the determination of urinary cortisol has proved to be a valuable parameter in evaluation of adrenocortical function. We compared the procedures of cortisol determination by radioimmunoassay after extraction and without extraction direct in crude urine. Direct determination needs less tracer and antibody and is less laborious. Intra- and interassay variance is 4.5%, 16.0% in the extraction method and 2.7%, 8.2% in the direct method, respectively. Comparison of both methods is acceptable (r = 0.92), recovery and dilution studies prove the direct method to be correct. Despite a lowered specificity, which gives higher results (factor 1.7) compared to the extraction method, the direct method is entirely sufficient for routine laboratory purposes. It can be achieved without additional equipment by laboratories, which determine total cortisol concentration in plasma.