A 22-year-old man passes 8 litres of urine daily and drinks compulsively. During a water deprivation test his plasma osmolality rises to 300 mOsm/kg but urine osmolality remains at 110 mOsm/kg. Two hours after subcutaneous desmopressin, urine osmolality rises to 480 mOsm/kg. The most likely diagnosis is:

• A Nephrogenic diabetes insipidus
• B Primary polydipsia
• C Central diabetes insipidus ✓
• D SIADH

Explanation

Failure to concentrate urine despite adequate dehydration proves a concentrating defect, and a rise in urine osmolality of more than 50 percent after desmoprosin indicates responsiveness to vasopressin, localising the defect to the hypothalamo-neurohypophyseal axis, i.e. central diabetes insipidus. In nephrogenic DI the kidney cannot respond, so desmopressin produces no significant rise. In primary polydipsia, urine concentrates during the deprivation phase itself, before desmopressin is given.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.