The most appropriate diagnostic test to perform next is a CT scan of the adrenal glands. This patient was suspected to have Cushing syndrome (CS) based on pathognomonic clinical findings and is not taking exogenous glucocorticoids. Two screening tests for CS are abnormal, which is adequate to establish the diagnosis. The next step in evaluation is measurement of the plasma adrenocorticotropic hormone (ACTH) level to differentiate between ACTH-dependent and ACTH-independent CS. With ACTH-dependent causes, the plasma ACTH is usually greater than 20 pg/mL (4.4 pmol/L). However, the low plasma ACTH reported in this patient is consistent with ACTH-independent CS. A CT scan of the abdomen to evaluate the adrenal glands is therefore indicated because the most common causes of ACTH-independent CS are adenomas and carcinomas of the adrenal cortex.

Inferior petrosal sinus sampling and MRI of the pituitary gland are used to confirm the presence of a corticotroph adenoma of the pituitary gland in ACTH-dependent CS, which is the most common cause of CS overall. However, neither is indicated in this patient because biochemical testing has not revealed ACTH-dependent CS.

The diagnosis of CS requires that at least two first-line screening tests be abnormal, including the low-dose dexamethasone suppression test (LDST) (both standard and overnight), 24-hour urine free cortisol (UFC), or late-night salivary cortisol. This patient failed to suppress cortisol levels following an overnight LDST and had an elevated 24-hour UFC level confirmed by a repeat collection. Therefore, measurement of late night salivary cortisol is unnecessary because the diagnosis of CS has already been established.