A patient with diabetes insipidus (central) has serum osmolality of 305 mOsm/kg and urine osmolality of 80 mOsm/kg. After administration of desmopressin (DDAVP), urine osmolality rises to 650 mOsm/kg. Which conclusion is correct?

• A Nephrogenic diabetes insipidus — kidneys fail to respond to ADH or DDAVP
• B Central (neurogenic) diabetes insipidus — the kidney can concentrate urine when exogenous ADH/DDAVP is given, confirming the defect is in ADH synthesis or secretion from hypothalamus/posterior pituitary ✓
• C Primary polydipsia — DDAVP has no role in differentiating from DI
• D SIADH — the water deprivation test with DDAVP would be expected to cause hyponatremia

Explanation

In the water deprivation + DDAVP test, a rise in urine osmolality of >50% after DDAVP confirms central DI — the renal collecting ducts are intact and respond normally to exogenous ADH. In nephrogenic DI, the kidneys fail to respond to DDAVP (urine osmolality remains < 300 mOsm/kg or rises < 10%). In primary polydipsia, urine concentrates adequately after water deprivation alone (before DDAVP). This distinction is critical for targeted therapy: central DI is treated with DDAVP; nephrogenic DI requires thiazides, amiloride, or indomethacin.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.