Urinary-free cortisol-based thresholds for differentiating ACTH-dependent Cushing: a Spanish validation study.

CONTEXT: 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.