Test of choice for diagnosing cushings syndrome: September 2011
**Core Concept**
Cushing's syndrome is a hormonal disorder caused by excess levels of cortisol in the body, resulting from various etiologies including endogenous production, exogenous administration, or ectopic production. The diagnosis of Cushing's syndrome involves identifying the underlying cause and assessing the clinical manifestations of hypercortisolism.
**Why the Correct Answer is Right**
The overnight dexamethasone suppression test (ODST) is a sensitive test for diagnosing Cushing's syndrome. It involves administering a low dose of dexamethasone, a synthetic glucocorticoid, at bedtime. Normally, dexamethasone suppresses the production of cortisol. However, in patients with Cushing's syndrome, the cortisol levels remain elevated even after dexamethasone administration. This is because the excess cortisol production is not suppressed by the negative feedback mechanism.
**Why Each Wrong Option is Incorrect**
**Option A:** The 24-hour urine free cortisol (UFC) test is a more sensitive test for diagnosing Cushing's syndrome but is not the test of choice for initial evaluation.
**Option B:** The late-night salivary cortisol test is a non-invasive test but has limited sensitivity and specificity.
**Option C:** The high-dose dexamethasone suppression test (HDDST) is used to distinguish between ACTH-dependent and ACTH-independent Cushing's syndrome but is not the initial test of choice.
**Clinical Pearl / High-Yield Fact**
The overnight dexamethasone suppression test is a useful initial screening test for Cushing's syndrome, but a high index of suspicion and a thorough clinical evaluation are necessary to make an accurate diagnosis.
**Correct Answer:** A. Overnight dexamethasone suppression test