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Difficulty: HardOrdering Laboratory Tests

A 38-year-old woman is evaluated in the clinic for progressive weight gain, severe proximal muscle weakness, and easy bruising over the past 14 months. Blood pressure is 158/98 mm Hg. Physical examination reveals pronounced central adiposity, facial plethora, and wide, violaceous abdominal striae. Basic laboratory testing shows a fasting blood glucose of 145 mg/dL. The physician suspects Cushing syndrome and plans a comprehensive biochemical diagnostic workup.

Arrange the following diagnostic tests in the most appropriate sequential order to establish the diagnosis and etiology of this patient's condition, starting with the initial screening step and concluding with the definitive invasive localization test.

  1. 124-hour urine free cortisol excretion
  2. 2Plasma adrenocorticotropic hormone (ACTH) concentration
  3. 3High-dose (8-mg) dexamethasone suppression test
  4. 4Inferior petrosal sinus sampling (IPSS)

Answer

The correct sequence begins with establishing the presence of hypercortisolism (24-hour urine free cortisol), followed by determining ACTH dependence (Plasma ACTH), differentiating the source of ACTH (High-dose dexamethasone suppression test), and finally confirming the pituitary source definitively (Inferior petrosal sinus sampling).
The diagnostic evaluation of Cushing syndrome strictly follows a step-wise physiological logic to minimize false positives and avoid unnecessary invasive procedures. The correct sequence is: 1) Establish hypercortisolism (24-hour urine free cortisol) to confirm the disease state. 2) Determine ACTH-dependence (Plasma ACTH) to separate adrenal (ACTH-independent) from central/ectopic (ACTH-dependent) etiologies. 3) Differentiate the ACTH source non-invasively (High-dose dexamethasone suppression test), as pituitary sources typically suppress while ectopic sources do not. 4) Perform invasive localization (Inferior petrosal sinus sampling) only when non-invasive tests and imaging fail to clearly identify the source.

Step-by-Step Solution

1
Select the initial screening test for hypercortisolism.
24-hour urine free cortisol excretion.
Before localizing a lesion, biochemical evidence of pathological hypercortisolism must be definitively established using 24-hour urine free cortisol, late-night salivary cortisol, or a 1-mg overnight dexamethasone suppression test.
2
Select the test to categorize the hypercortisolism.
Plasma adrenocorticotropic hormone (ACTH) concentration.
A suppressed ACTH level indicates an ACTH-independent adrenal source, while a normal or elevated ACTH level indicates an ACTH-dependent source (pituitary or ectopic).
3
Select the non-invasive test to differentiate ACTH-dependent sources.
High-dose (8-mg) dexamethasone suppression test.
Pituitary adenomas causing Cushing disease typically retain some negative feedback sensitivity and will suppress cortisol production with high-dose dexamethasone, whereas ectopic ACTH tumors do not.
4
Select the definitive localization test for challenging cases.
Inferior petrosal sinus sampling (IPSS).
If non-invasive biochemical tests and MRI are equivocal or discordant, IPSS is used to measure the central-to-peripheral ACTH gradient to definitively confirm or rule out a pituitary source.

Key Concept

Diagnostic algorithm and test sequencing for Cushing syndrome.
Estimated Time:1m 30s
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Topics

Ordering Laboratory Tests

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