A 20-year-old man presents with polyuria (8 L/day), polydipsia, and nocturia. Urine osmolality after overnight fast is 95 mOsm/kg. After IV desmopressin (DDAVP) 2 μg, urine osmolality rises to 680 mOsm/kg. Plasma osmolality is 299 mOsm/kg. What is the diagnosis?
- A Nephrogenic diabetes insipidus
- B Primary polydipsia (psychogenic)
- C Central diabetes insipidus ✓
- D Osmotic diuresis
Explanation
In the water deprivation test interpretation: urine osmolality <300 mOsm/kg after fluid deprivation (confirming inability to concentrate urine), combined with a brisk >50% rise in urine osmolality (here from 95 to 680 mOsm/kg — a 616% increase) after exogenous DDAVP, is diagnostic of central (neurogenic) diabetes insipidus — the kidney can concentrate urine normally in response to exogenous ADH. Nephrogenic DI shows <50% or minimal rise after DDAVP. Primary polydipsia usually shows some concentration ability after fluid deprivation (urine Osm 200–400) and does not show such dramatic DDAVP response.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.