What diagnostic test is used to confirm suspected Cushing's syndrome?

What diagnostic test is used to confirm suspected Cushing's syndrome?

• A. Dexamethasone suppression test
• B. Late-night salivary cortisol
• C. 24-hour urinary free cortisol
• D. Serum ACTH level

Answer: (C)

When confirming suspected Cushing's syndrome, you want a test that shows ongoing, excessive cortisol production rather than a snapshot value. The 24-hour urinary free cortisol does just that by measuring all cortisol excreted in urine over an entire day, capturing the true level of production despite normal diurnal fluctuations. An elevated 24-hour UFC provides solid evidence of hypercortisolism and typically prompts further workup to identify the underlying source (pituitary, adrenal, or ectopic). Dexamethasone suppression tests are commonly used questions for screening and initial assessment of cortisol regulation, because they test whether cortisol production can be suppressed by steroids; a lack of suppression suggests Cushing's, but the result is not as direct a measure of total daily cortisol output as the UFC reflects. Late-night salivary cortisol is another screening approach that relies on loss of normal diurnal variation, but it serves more as a screening tool than a definitive confirmation. Serum ACTH helps differentiate ACTH-dependent from ACTH-independent causes after confirming hypercortisolism, not in establishing that cortisol excess is present.

When confirming suspected Cushing's syndrome, you want a test that shows ongoing, excessive cortisol production rather than a snapshot value. The 24-hour urinary free cortisol does just that by measuring all cortisol excreted in urine over an entire day, capturing the true level of production despite normal diurnal fluctuations. An elevated 24-hour UFC provides solid evidence of hypercortisolism and typically prompts further workup to identify the underlying source (pituitary, adrenal, or ectopic).

Dexamethasone suppression tests are commonly used questions for screening and initial assessment of cortisol regulation, because they test whether cortisol production can be suppressed by steroids; a lack of suppression suggests Cushing's, but the result is not as direct a measure of total daily cortisol output as the UFC reflects. Late-night salivary cortisol is another screening approach that relies on loss of normal diurnal variation, but it serves more as a screening tool than a definitive confirmation. Serum ACTH helps differentiate ACTH-dependent from ACTH-independent causes after confirming hypercortisolism, not in establishing that cortisol excess is present.