Biagetti, BetinaMarques, PedroSoto-Moreno, AlfonsoGarcía-Centeno, RogelioGonzález-Fernández, LauraGarcia, María Dolores OlleroEcharri, Ana IrigarayCardona-Arias, AndresRodríguez, Maria Dolores MourePaja, MiguelCastro, AnaValero, Lucía ManzanoGuerrero-Pérez, FernandoLamas, CristinaLázaro, Victoria AlcázarGracia, PaolaSanchis-Pascual, DavidÁLvarez-Escolá, CristinaLozano-Aida, ClaudiaHanzu, Felicia A.Araujo-Castro, Marta2026-05-212026-05-212026-04-309654b801-7417-4ea7-9e49-84106c593647http://hdl.handle.net/10400.14/57798Context: Differentiating ectopic ACTH secretion (EAS) from Cushing disease (CD) remains one of the most challenging steps in the diagnostic workup of ACTH-dependent Cushing syndrome (CS). Urinary-free cortisol (UFC) expressed as times above the upper limit of normal (ULN) has been proposed as a simple, noninvasive discriminator, but external validation in independent populations is lacking. Objective: To validate the diagnostic performance of UFC × ULN for distinguishing EAS from CD and explore complementary biochemical markers, including late-night salivary cortisol (LNSC × ULN) and hypokalemia. Design, Setting, and Participants Multicenter retrospective study from the Spanish Cushing Registry including 269 patients with ACTH-dependent Cushing’s syndrome (208 CD, 61 EAS) diagnosed and managed in tertiary referral centers. Main Outcome Measures: Diagnostic accuracy of UFC × ULN and LNSC × ULN for discriminating EAS from CD, expressed as area under the ROC curve (AUC), sensitivity, specificity, and predictive value. Results: EAS patients were older (median 59.0 vs 44.9 years; P < .001) and showed higher UFC × ULN (16.6 vs 3.6; P < .001) and LNSC × ULN (9.3 vs 1.5; P < .001). UFC × ULN and LNSC × ULN achieved excellent discriminative performance (AUC 0.90 and 0.92). No EAS occurred with UFC × ULN < 3 × ULN, while 40.5% of patients with UFC ≥ 10 × ULN had EAS. The combination of severe hypercortisolism (UFC ≥ 10 × ULN and LNSC ≥ 9 × ULN) plus hypokalemia identified 75% of EAS with 98% specificity. Conclusion: UFC × ULN thresholds reliably stratify the probability of EAS vs CD. Severe hypercortisolism and hypokalemia strongly predict EAS, supporting a pragmatic diagnostic approach that prioritizes whole-body imaging in high-risk patients and pituitary-centered evaluation in mild cases.engCushing diseaseCushing syndromeEctopic cushingHypercortisolismUrinary-free cortisolresearch article10.1210/clinem/dgag18742063386001760279200001