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Difficulty: MediumDiagnostic Workup and Test Sequencing

An adult female patient is evaluated in the outpatient clinic for a recent history of progressive weight gain, easy bruising, and proximal muscle weakness. Physical examination reveals facial plethora, supraclavicular fat pads, and violaceous striae on the lower abdomen. Her blood pressure is 154/96 mm Hg. The physician strongly suspects endogenous hypercortisolism (Cushing syndrome) and plans a stepwise evaluation to confirm the diagnosis and localize the etiology. Arrange the following diagnostic steps in the most appropriate sequential order, from initial screening to final invasive localization.

  1. 1Measurement of 24-hour urine free cortisol
  2. 2Measurement of plasma adrenocorticotropic hormone (ACTH) level
  3. 3High-dose dexamethasone suppression test
  4. 4Magnetic resonance imaging (MRI) of the pituitary gland
  5. 5Bilateral inferior petrosal sinus sampling

Answer

The correct sequence is: Measurement of 24-hour urine free cortisol -> Measurement of plasma adrenocorticotropic hormone (ACTH) level -> High-dose dexamethasone suppression test -> Magnetic resonance imaging (MRI) of the pituitary gland -> Bilateral inferior petrosal sinus sampling.
The diagnostic pathway for Cushing syndrome must follow a strict logical sequence to prevent misdiagnosis. First, endogenous hypercortisolism must be confirmed (e.g., via 24-hour urine free cortisol). Once confirmed, a plasma ACTH level determines if the condition is ACTH-dependent or ACTH-independent. For ACTH-dependent cases, a high-dose dexamethasone suppression test helps differentiate a pituitary source from an ectopic source. Only after this biochemical evidence points to the pituitary gland should an MRI be performed. Finally, if the MRI is negative or reveals a lesion smaller than 6 mm, bilateral inferior petrosal sinus sampling is indicated to definitively confirm the presence of an ACTH-secreting pituitary microadenoma before proceeding to surgery.

Step-by-Step Solution

1
Confirm the presence of hypercortisolism.
Measurement of 24-hour urine free cortisol (or low-dose dexamethasone suppression test / late-night salivary cortisol).
Establishing the diagnosis of Cushing syndrome is the mandatory first step to avoid false positives.
2
Determine if the hypercortisolism is ACTH-dependent or independent.
Measurement of plasma ACTH level.
This categorizes the etiology into central/ectopic (ACTH-dependent) versus adrenal (ACTH-independent).
3
Differentiate the source of ACTH-dependent hypercortisolism.
High-dose dexamethasone suppression test.
Pituitary adenomas typically suppress cortisol production with high-dose dexamethasone, whereas ectopic ACTH tumors usually do not.
4
Anatomically localize the suspected tumor.
Magnetic resonance imaging (MRI) of the pituitary gland.
Biochemical testing must direct imaging to minimize the risk of identifying non-functioning incidentalomas.
5
Perform invasive confirmatory testing if imaging is inconclusive.
Bilateral inferior petrosal sinus sampling.
If the MRI is negative or shows a lesion smaller than 6 mm, IPSS is required to definitively confirm a pituitary source before neurosurgery.

Key Concept

The diagnostic workup of Cushing syndrome follows a strict tier: confirm hypercortisolism, establish ACTH dependence, differentiate the ACTH source biochemically, localize the lesion anatomically, and pursue invasive sampling only if non-invasive localization is equivocal.
Estimated Time:1m 30s
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Diagnostic Workup and Test Sequencing

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