Urinary free cortisol analyses: Enhancing their clinical performance in Cushing's syndrome management by means of LC-MS/MS.

24 h urinary free cortisol (UFC) analysis constitutes one of the three first level recommended tests in Cushing's syndrome (CS) diagnostic confirmation work up. However, it occasionally leads to inaccurate results due to the use of immunoassays (IAs) or the concomitant administration of exogenous glucocorticoids, among others. This study aimed to develop a rapid and accurate LC-MS/MS method which may ultimately replace the use of IAs, and also provide relevant clinical information through the simultaneous monitoring of UFC, cortisone, and dexamethasone. An LC-MS/MS method based on direct injection approach was developed and fully characterized for the quantitation of the target analytes. A population-based reference range was established, and the potential supporting role of cortisone and cortisol/cortisone ratio was comprehensively assessed in patients under CS follow-up or clinical suspicion for hypercortisolism. The presence of dexamethasone was also assessed in order to exclude invalid samples from evaluation. Significant differences were observed for cortisone and cortisol/cortisone ratio between the control group and patients with hyper-/hypocortisolism, and an ideal level of biochemical agreement was observed with UFC LC-MS/MS values when the combination of both biomarkers was considered. Dexamethasone was detected in up to 7.7% of the studied population. The herein presented LC-MS/MS approach not only offers the possibility of discontinuing the use of IAs, but also provides additional biomarkers which are significantly relevant in CS management, thus enhancing the overall clinical performance of UFC analyses.