Urine free cortisol analysis by automated immunoassay and high-performance liquid chromatography for the investigation of Cushing's syndrome

Abstract

Background: We compared a widely used immunoassay method for urine free corticoids with a high-performance liquid chromatography (HPLC) method for the investigation of Cushing's syndrome. Methods: Urine free corticoid analysis using an Immulite:2000 automated immunoassay analyser and urine free cortisol analysis by a specific HPLC method on 39 patients with an elevated baseline immunoassay 24 h urine free corticoid result who were referred from 2000 to 2004. Results: Reference ranges for urine free corticoids by immunoassay and for urine free cortisol by HPLC were <900 and <170 nmol/day, respectively. Of the 39 patients with an elevated immunoassay urine free corticoid result, Cushing's syndrome was surgically proven in 22 patients, giving a positive predictive value of 56%. Cushing's syndrome was excluded in 12 of the remaining 17 patients by applying the specific HPLC method and at least one of the following: a clinical evaluation which was not suggestive or adequate suppression of plasma cortisol following dexamethasone. Urine free cortisol analysis by HPLC gave a positive predictive value of 81% for Cushing's syndrome. Conclusions: Although immunoassay has a low specificity for urine cortisol, it is a widely available test which serves as a valid initial investigation of Cushing's syndrome.