Identification of patients with Cushing's disease with negative pituitary adrenocorticotropin gradients during inferior petrosal sinus sampling: prolactin as an index of pituitary venous effluent.

Inferior petrosal sinus sampling for ACTH differentiates pituitary ACTH-dependent Cushing's (CD) from the ectopic ACTH syndrome (EAS). Petrosal sinus to peripheral (IPS:P) ACTH ratios greater than 2.0 in the basal state or a peak greater than 3.