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Difficulty: HardOrdering Laboratory Tests

A 42-year-old woman is evaluated in the outpatient clinic for a 6-month history of progressive weight gain, easy bruising, and proximal muscle weakness. Physical examination reveals central adiposity, facial plethora, wide purple striae on her abdomen, and trace lower extremity edema. Her blood pressure is 155/95 mmHg. She takes no medications and denies the use of exogenous glucocorticoids or over-the-counter supplements. The physician suspects an endogenous endocrine disorder and plans a comprehensive diagnostic workup.

Arrange the following diagnostic laboratory tests and procedures in the most appropriate sequential order to establish the definitive diagnosis and etiology for this patient, starting with the initial diagnostic screening step.

  1. 1Measurement of 24-hour urine free cortisol or late-night salivary cortisol
  2. 2Measurement of morning plasma adrenocorticotropic hormone (ACTH) concentration
  3. 3Administration of a high-dose (8 mg) dexamethasone suppression test
  4. 4Performance of inferior petrosal sinus sampling (IPSS) for ACTH

Answer

The correct sequence is: 1) Measurement of 24-hour urine free cortisol or late-night salivary cortisol, 2) Measurement of morning plasma ACTH concentration, 3) Administration of a high-dose dexamethasone suppression test, and 4) Performance of inferior petrosal sinus sampling (IPSS).
The diagnostic algorithm for Cushing syndrome is inherently sequential to minimize false positives, reduce unnecessary invasive testing, and accurately localize the pathology. It begins by establishing the diagnosis with a screening test (e.g., 24-hour urine free cortisol). Next, ACTH is measured to distinguish between adrenal (ACTH-independent) and pituitary/ectopic (ACTH-dependent) etiologies. If ACTH-dependent, a high-dose dexamethasone suppression test evaluates the source, as pituitary adenomas typically retain some negative feedback response, whereas ectopic tumors do not. Finally, inferior petrosal sinus sampling is reserved as an invasive, definitive localizing step if non-invasive tests and imaging are equivocal.

Step-by-Step Solution

1
Confirm the presence of hypercortisolism.
Measurement of 24-hour urine free cortisol or late-night salivary cortisol.
The first step in evaluating suspected endogenous Cushing syndrome is to biochemically establish that the patient has pathological hypercortisolism, ruling out normal physiological variations.
2
Determine the ACTH dependence of the hypercortisolism.
Measurement of morning plasma ACTH concentration.
Once hypercortisolism is confirmed, ACTH levels differentiate between adrenal causes (low ACTH due to negative feedback) and ACTH-dependent causes (normal or elevated ACTH).
3
Differentiate the source of ACTH secretion.
Administration of a high-dose dexamethasone suppression test.
In ACTH-dependent hypercortisolism, a high-dose dexamethasone test helps distinguish a pituitary adenoma (which typically suppresses) from ectopic ACTH secretion (which typically does not suppress).
4
Definitively localize the ACTH source if non-invasive workup is indeterminate.
Performance of inferior petrosal sinus sampling (IPSS).
If biochemical tests and imaging are conflicting, IPSS is the gold standard invasive test to confirm a pituitary source of ACTH before proceeding to neurosurgery.

Key Concept

The diagnostic evaluation of Cushing syndrome follows a strict sequence: confirm hypercortisolism, establish ACTH dependence, and then localize the anatomical source.
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Ordering Laboratory Tests

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