A patient with an ACTH-secreting pituitary adenoma (Cushing disease) undergoes inferior petrosal sinus sampling (IPSS). The IPSS central-to-peripheral ACTH ratio is >2 at baseline and >3 after CRH stimulation. This test is used to:

• A Distinguish pituitary source from ectopic ACTH production by demonstrating gradient from petrosal sinus draining the pituitary ✓
• B Localize the exact side of the pituitary microadenoma to guide transsphenoidal surgery
• C Confirm cortisol hypersecretion by demonstrating failure of dexamethasone suppression
• D Differentiate adrenal adenoma from Cushing disease by measuring peripheral ACTH levels

Explanation

IPSS measures ACTH in the inferior petrosal sinuses (which drain the pituitary) and in peripheral blood simultaneously, before and after CRH stimulation. A central:peripheral ACTH ratio >2 basally or >3 after CRH confirms that the ACTH source is the pituitary gland (Cushing disease), while a ratio <2 suggests ectopic ACTH production (e.g., from a small cell lung cancer). Ectopic ACTH-secreting tumors typically do not respond to CRH, keeping the ratio low. Side-to-side petrosal sinus comparison can help lateralize the adenoma but has only ~70% accuracy for surgical guidance.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.