Prolactin-adjusted inferior petrosal sinus sampling: Pituitary and ectopic adrenocorticotropic hormone-dependent Cushing syndrome.

Inferior petrosal sinus sampling (IPSS) is a diagnostic procedure used to differentiate between ectopic adrenocorticotropic hormone (ACTH)-dependent Cushing syndrome (EAS) and pituitary ACTH-dependent Cushing syndrome (CD). This study investigated the diagnostic value of IPSS, focusing on the use of prolactin adjustments and different calculation methods. We retrospectively analyzed data from patients with ACTH-dependent Cushing syndrome and inconclusive pituitary-MRI who underwent IPSS with corticotropin-releasing hormone (CRH) stimulation between 2015 and 2025. The cohort included 19 patients (16 CD, 3 EAS), with diagnoses confirmed by pathology examination and/or biochemical remission 1 year post-surgery. A pituitary source was confirmed in all patients with CD (n = 16) through pathology and/or biochemical remission. An ectopic source was confirmed by pathology in two of three patients with EAS. Using unadjusted ACTH ratios and previously established cut-off values resulted in three incorrect diagnoses out of 20 procedures. In contrast, prolactin-adjusted peak ACTH ratios provided a more distinct separation between CD and EAS, enabling correct diagnosis in all cases. Optimal cut-off values determined by receiver operating characteristic curve analysis were 1.0 for basal and 1.7 for concurrent prolactin-adjusted peak ACTH ratios, yielding 100% sensitivity and specificity. Basal prolactin-adjusted peak ACTH ratios were >1.5 in all patients with CD and <0.6 in all patients with EAS, while concurrent ratios were >1.1 in all patients with CD and <0.3 in all patients with EAS. Prolactin-adjusted peak ACTH ratios improve the diagnostic accuracy of IPSS and can effectively differentiate between ectopic and pituitary sources of ACTH. This study enhances the diagnostic accuracy of inferior petrosal sinus sampling (IPSS) for differentiating pituitary from ectopic ACTH-dependent Cushing syndrome by incorporating prolactin measurements and exploring various calculation methods. The findings contribute to advancing diagnostic techniques and improving clinical management of endocrine disorders. By enabling more accurate identification of the underlying cause of ACTH-dependent Cushing syndrome, this work supports clinicians in selecting optimal treatment strategies.