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Difficulty: HardOrdering Laboratory Tests

A 44-year-old female is evaluated in the ambulatory care clinic due to progressive weight gain, easy bruising, and profound proximal muscle weakness over the past eight months. Physical examination is notable for facial plethora, prominent supraclavicular fat pads, and broad purplish abdominal striae. Her blood pressure is 158/96 mmHg. She has no history of exogenous glucocorticoid use. The physician suspects endogenous hypercortisolism.

Arrange the following diagnostic laboratory tests and procedures in the most appropriate sequential order to establish the diagnosis and identify the precise etiology.

  1. 1Quantification of 24-hour urine free cortisol or late-night salivary cortisol
  2. 2Measurement of baseline plasma adrenocorticotropic hormone (ACTH) concentration
  3. 3High-dose (8-mg) dexamethasone suppression test
  4. 4Bilateral inferior petrosal sinus sampling (BIPSS)

Answer

The correct sequence is: 1) Quantification of 24-hour urine free cortisol or late-night salivary cortisol, 2) Measurement of baseline plasma ACTH concentration, 3) High-dose (8-mg) dexamethasone suppression test, 4) Bilateral inferior petrosal sinus sampling (BIPSS).
Establishing the diagnosis and etiology of Cushing syndrome requires a strict sequential laboratory approach to avoid misdiagnosis caused by incidentalomas or physiological variations. The required sequence begins with confirming pathological hypersecretion of cortisol via 24-hour urine free cortisol, late-night salivary cortisol, or a low-dose dexamethasone suppression test. Only after hypercortisolism is verified should the physician measure plasma ACTH to categorize the syndrome as ACTH-dependent or ACTH-independent. If it is ACTH-dependent, the next step is determining the exact source (pituitary versus ectopic) using a high-dose (8-mg) dexamethasone suppression test. Finally, if non-invasive tests and imaging are contradictory or inconclusive, bilateral inferior petrosal sinus sampling (BIPSS) is the definitive diagnostic procedure to confirm a pituitary source.

Step-by-Step Solution

1
Confirm the presence of hypercortisolism.
Order a 24-hour urine free cortisol, late-night salivary cortisol, or 1-mg overnight dexamethasone suppression test.
Before attempting to localize a lesion, the diagnosis of Cushing syndrome must be biochemically confirmed to rule out physiologic variants or misdiagnosis.
2
Determine ACTH dependence.
Measure baseline plasma ACTH levels.
A suppressed ACTH level indicates an adrenal source (ACTH-independent), whereas a normal or elevated ACTH level indicates a pituitary or ectopic source (ACTH-dependent).
3
Differentiate the source of ACTH production.
Perform a high-dose dexamethasone suppression test.
Pituitary adenomas typically retain some sensitivity to glucocorticoid negative feedback and will suppress with high-dose dexamethasone, unlike most ectopic ACTH-producing tumors which will not suppress.
4
Definitively localize the ACTH source if non-invasive tests are equivocal.
Perform bilateral inferior petrosal sinus sampling (BIPSS).
If biochemical tests and pituitary MRI are discordant, BIPSS is utilized to directly measure the central-to-peripheral ACTH gradient to definitively diagnose Cushing disease versus ectopic ACTH syndrome.

Key Concept

The diagnostic sequencing for Cushing syndrome involves a strict stepwise approach: confirm pathological hypercortisolism first, determine ACTH dependence second, and localize the source last.
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