During a routine occupational health evaluation, a 34-year-old female intensive care unit … — USMLE Step 3

During a routine occupational health evaluation, a 34-year-old female intensive care unit nurse complains of progressive central adiposity, pronounced muscle weakness when climbing stairs, and easy bruising over the past eight months. She currently works exclusively on nocturnal shifts ( $7:00\text{ PM}$ to $7:00\text{ AM}$ ). Her medical history includes polycystic ovary syndrome, for which she takes a daily combined oral contraceptive pill. Vital signs show a blood pressure of $158/96\text{ mmHg}$ and a pulse of $82/\text{min}$ . Her body mass index is $34\text{ kg/m}^2$ . Physical examination reveals facial plethora, dorsal cervical and supraclavicular fat pad enlargement, and wide, violaceous striae on the lower abdomen. Which of the following is the most appropriate initial laboratory test to screen for this patient's suspected condition?

24-hour urine free cortisol excretionAnswer
B
1-mg overnight dexamethasone suppression test
C
Late-night salivary cortisol
D
Serum adrenocorticotropic hormone (ACTH) level
E
8:00 AM serum cortisol level

Answer

The most appropriate test is 24-hour urine free cortisol excretion, as it avoids interference from both the patient's disrupted sleep-wake cycle and her oral contraceptive use.

The correct answer identifies the 24-hour urine free cortisol as the only valid screening test for this patient. While there are three acceptable initial screening tests for Cushing syndrome (24-hour urine free cortisol, late-night salivary cortisol, and 1-mg overnight dexamethasone suppression), the patient's unique history eliminates two of them. Her nocturnal work schedule disrupts her normal circadian rhythm, rendering late-night salivary cortisol highly inaccurate. Furthermore, she takes an oral combined hormonal contraceptive; the estrogen component heavily stimulates hepatic production of cortisol-binding globulin (CBG). Because the 1-mg overnight dexamethasone suppression test measures total serum cortisol rather than free cortisol, the expanded CBG pool holds excess bound cortisol, resulting in a false-positive failure to suppress. The 24-hour urine collection directly measures free, active cortisol and integrates the values over a full day, entirely bypassing the confounding variables of altered CBG and disrupted diurnal variation.

Step-by-Step Solution

Identify the suspected clinical syndrome based on the patient's presentation.

The patient's central adiposity, proximal muscle weakness, easy bruising, wide violaceous striae, and hypertension are classic signs of Cushing syndrome (hypercortisolism).

Recognizing the syndrome dictates the diagnostic algorithm, which begins with biochemical screening for excess cortisol.

Recall the three standard first-line screening tests for Cushing syndrome.

The standard tests are 24-hour urine free cortisol, late-night salivary cortisol, and the 1-mg overnight dexamethasone suppression test.

Any of these three tests can normally be used as an initial screening tool in the general population.

Evaluate patient-specific factors that would invalidate any of the standard screening tests.

The patient is a nocturnal shift worker, which disrupts normal diurnal cortisol variation, invalidating late-night salivary cortisol. She also takes oral contraceptives (estrogen), which increase cortisol-binding globulin (CBG), leading to false positives on the 1-mg dexamethasone suppression test.

Selecting the correct laboratory test requires integrating the patient's pharmacological and social history to avoid false-positive results.

Determine the remaining valid screening test.

The 24-hour urine free cortisol measurement assesses unbound cortisol over an entire day and is unaffected by both CBG fluctuations and diurnal rhythm shifts.

It is the only reliable diagnostic option left for this specific patient profile.

Key Concept

Selecting the appropriate initial screening test for Cushing syndrome requires adjusting for factors that alter cortisol-binding globulin (e.g., oral estrogens) and circadian rhythm (e.g., shift work).

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