Modest sensitivity of 24 h urine free cortisol LC-MS/MS to detect Cushing's disease

PURPOSE: Cushing's disease (CD) is a rare disorder of endogenous hypercortisolism arising from a corticotroph adenoma and is associated with significant morbidity and mortality. Measurement of 24-h urine free cortisol (UFC) by liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a recommended first-line screening test. We aimed to evaluate the diagnostic sensitivity of newly established, sex-specific UFC LC-MS/MS reference ranges in a surgical cohort of patients with CD. METHODS: We performed retrospective analysis of adults undergoing pituitary surgery for confirmed CD at a tertiary center between June 2021 and July 2025. All patients met clinical, biochemical, and radiological criteria for CD. UFC was quantified using an accredited LC-MS/MS assay, according to recently published, sex-specific, optimized reference intervals. Sensitivity of both urine volume-corrected (VC-UFC; nmol/day) and uncorrected (U-UFC; nmol/L) reference ranges were assessed. RESULTS: Sixteen patients (UFC measurements: 28 measurements from 13 females and 8 measurements from 3 males) met inclusion criteria; 88% had histologic confirmation of corticotroph adenoma. Sensitivity of any single UFC measure for CD was 78% (95% CI 61% to 90%); 75% for males (95% CI 25% to 97%) and 79% for females (95% CI 59% to 92%) for VC-UFC and 56% (95% CI 38% to 72%); 88% for males (95% CI 47% to 100%) and 46% for females (95% CI 28% to 66%) for U-UFC. CONCLUSION: Despite use of LC-MS/MS and rigorously validated reference ranges, UFC demonstrated only modest sensitivity for detecting CD. These findings underscore the limitations of single UFC as a screening test and support the need for multiple collections and complementary biochemical assessments in suspected CD.