For initial screening of cushing syndrome which one among following is

For initial screening of cushing syndrome which one among following is diagnostic A • over right dexame thasme test B. Low dose dexama those test C. high dose Dexame shase test el• plasma ACTH determmation e. MRI brain

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"Cushing syndrome" AND "screening" AND "dexamethasone suppression test"

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Answer: B. Low Dose Dexamethasone Suppression Test (LDDST)

Why LDDST is the Screening Test of Choice

The low-dose dexamethasone suppression test (LDDST) is the standard initial screening test for Cushing syndrome. There are two forms used in practice:

1. Overnight (1 mg) DST - the most commonly used screening version:

1 mg dexamethasone is given at 11 PM
Serum cortisol is measured between 8-9 AM the next morning
In normal individuals, dexamethasone suppresses ACTH and cortisol (cortisol falls to <1.8 mcg/dL / 50 nmol/L)
In Cushing syndrome, cortisol is NOT suppressed

2. Classic 2-day LDDST (0.5 mg every 6 hours for 48 hours) - sensitivity of ~98% for Cushing syndrome diagnosis

Why the Other Options Are Wrong

Option	Role	Why Not for Screening
A. Overnight DST (1 mg)	This IS a form of LDDST - essentially the same test. Some MCQs treat "overnight DST" and "LDDST" as the correct answer together. If listed separately, LDDST (2-day) is more definitive
C. High Dose DST (8 mg)	Used for differentiation - suppresses pituitary Cushing disease but NOT adrenal or ectopic; not a screening test
D. Plasma ACTH	Used after Cushing is confirmed, to distinguish ACTH-dependent (pituitary/ectopic) vs. ACTH-independent (adrenal) causes
E. MRI Brain	Used only after biochemical confirmation - for localizing a pituitary adenoma

Stepwise Approach to Cushing Syndrome

STEP 1 - SCREENING (confirm hypercortisolism):
  • Low-dose DST (1 mg overnight OR 2-day 0.5 mg)      ← ANSWER
  • 24-hour urinary free cortisol
  • Late-night salivary cortisol

STEP 2 - DIFFERENTIATION (once confirmed):
  • Plasma ACTH level
    - ACTH high → ACTH-dependent (pituitary or ectopic)
    - ACTH low/suppressed → ACTH-independent (adrenal)

STEP 3 - LOCALIZATION:
  • High-dose DST (8 mg) - differentiates pituitary vs. ectopic
  • MRI pituitary / CT adrenals / CT chest

Key principle: Screening tests detect whether excess cortisol is present. The LDDST has ~95-98% sensitivity for this purpose - the highest among the listed options. Plasma ACTH and high-dose DST come into play only after Cushing syndrome has been biochemically confirmed.

Sources: Swanson's Family Medicine Review, p. 240; Smith and Tanagho's General Urology, 19th Ed., p. 526; Scott-Brown's Otorhinolaryngology, Vol. 1

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