During a water deprivation test, a patient's urine osmolality plateaus at 180 mOsm/kg despite a 3% rise in plasma sodium. One hour after subcutaneous desmopressin, urine osmolality rises to 520 mOsm/kg. What is the diagnosis?

• A Central diabetes insipidus ✓
• B Nephrogenic diabetes insipidus
• C Primary polydipsia
• D Syndrome of inappropriate antidiuretic hormone secretion

Explanation

Central diabetes insipidus shows inability to concentrate urine during dehydration followed by a brisk rise in urine osmolality, conventionally greater than 50%, after exogenous vasopressin, proving intact renal responsiveness to the hormone. In nephrogenic diabetes insipidus the kidney cannot respond, so desmopressin produces less than a 45% increment. In primary polydipsia maximal endogenous vasopressin already concentrates urine above 500 mOsm/kg during deprivation. SIADH causes concentrated urine and hyponatraemia, the opposite picture.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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