A woman with bluish discoloration, obesity, and hypertension is suspected of having Cushing's syndrome. Which test is best for confirmation?

In the context of confirming a diagnosis of Cushing's syndrome, the most reliable and specific test is the late-night salivary cortisol test. However, among the choices provided, the high-dose dexamethasone suppression test is particularly useful in distinguishing between different causes of Cushing's syndrome, particularly between a functioning adrenal adenoma and Cushing's disease due to pituitary adenomas.

The high-dose dexamethasone suppression test works by administering a larger dose of dexamethasone to see if cortisol production can be suppressed. If suppression occurs, it is more indicative of Cushing's disease (pituitary adenoma). If cortisol levels remain high, it suggests an adrenal source or ectopic ACTH secretion.

While the 24-hour urinary free cortisol is also a commonly used screening test for Cushing's syndrome, the high-dose dexamethasone suppression test provides additional insight necessary for accurate differentiation and confirmation when an adenoma is suspected.

In summary, the high-dose dexamethasone suppression test is best for confirming the etiology of Cushing's syndrome, particularly in distinguishing the source of the disorder when suspicion of pituitary involvement exists.