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Difficulty: EasyDiagnostic Workup and Test Sequencing

A 38-year-old woman is evaluated in the ambulatory clinic for a 6-month history of progressive weight gain, easy bruising, and worsening fatigue. She also reports that her menstrual periods have become irregular. Her blood pressure is 158/96 mmHg. Physical examination reveals central adiposity, facial plethora, a dorsocervical fat pad, and wide, violaceous striae on the lower abdomen. She takes no medications and has no history of exogenous glucocorticoid use. Which of the following is the most appropriate next step in the diagnostic workup?

  1. Overnight low-dose dexamethasone suppression testAnswer
  2. B
    Measurement of serum adrenocorticotropic hormone (ACTH)
  3. C
    Magnetic resonance imaging (MRI) of the pituitary gland
  4. D
    Computed tomography (CT) of the adrenal glands
  5. E
    Random morning serum cortisol measurement

Answer

Overnight low-dose dexamethasone suppression test
The patient presents with classic clinical features of Cushing syndrome. The diagnostic workup for Cushing syndrome must follow a strict sequence to avoid false-positive results. The first step is always to confirm the presence of endogenous hypercortisolism. Acceptable first-line screening tests include an overnight low-dose dexamethasone suppression test, a 24-hour urine free cortisol measurement, or a late-night salivary cortisol test. Only after hypercortisolism is biochemically confirmed should the clinician proceed to determine the etiology.

Step-by-Step Solution

1
Identify the clinical syndrome presented by the patient.
The patient has classical signs of Cushing syndrome, including central obesity, facial plethora, dorsocervical fat pad, and violaceous striae.
Recognizing the constellation of symptoms directs the specific diagnostic algorithm.
2
Determine the first step in the diagnostic sequence for this condition.
The initial step is to establish the presence of endogenous hypercortisolism using a highly sensitive screening test.
Confirmatory testing and localization must wait until the biochemical presence of the disease is established to avoid false positives.
3
Select the appropriate screening test from the available options.
An overnight low-dose dexamethasone suppression test is selected.
This is one of the three standard first-line screening tests for Cushing syndrome.

Key Concept

The initial diagnostic step for suspected Cushing syndrome requires confirming hypercortisolism with a first-line screening test (overnight low-dose dexamethasone suppression test, 24-hour urine free cortisol, or late-night salivary cortisol) before assessing ACTH levels or performing imaging.
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Diagnostic Workup and Test Sequencing

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