The diagnostic performance of urinary free cortisol is better than the cortisol:cortisone ratio in detecting de novo Cushing's syndrome: the use of a LC-MS/MS method in routine clinical practice.

Objective: The Endocrine Society Clinical Guidelines recommend measuring 24-h urinary free cortisol (UFF) levels using a highly accurate method as one of the first-line screening tests for the diagnosis of Cushing's Syndrome (CS). We evaluated the performance of UFF, urinary free cortisone (UFE), and the UFF:UFE ratio, measured using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method.

Subjects And Methods: The LC-MS/MS was used to analyze UFF and UFE levels in 43 surgically confirmed CS patients: 26 with Cushing's disease (CD, 16 de novo and ten recurrences), 11 with adrenal CS and six with ectopic CS; 22 CD patients in remission; 14 eu-cortisolemic CD patients receiving medical therapy; 60 non-CS patients; and 70 healthy controls. Sensitivity and specificity were determined in the combined groups of non-CS patients, healthy controls, and CD in remission.

Results: UFF>170 nmol/24 h showed 98.7% specificity and 100% sensitivity for de novo CS, while sensitivity was 80% for recurrent CD patients, who were characterized by lower UFF levels. The UFF:UFE and UFF+UFE showed lower sensitivity and specificity than UFF. Ectopic CS patients had the highest UFF and UFF:UFE levels, which were normal in the CD remission patients and in those receiving medical therapy.

Conclusions: Our data suggest high diagnostic performance of UFF excretion measured using LC-MS/MS, in detecting de novo CS. UFF:UFE and UFF+UFE assessments are not useful in the first step of CS diagnosis, although high levels were found to be indicative of ectopic CS.