After 8 hours of water deprivation, a patient has plasma osmolality of 305 mOsm/kg, urine osmolality of 120 mOsm/kg, and high plasma ADH. Administration of desmopressin raises urine osmolality to only 150 mOsm/kg. The diagnosis is:

• A Complete central diabetes insipidus
• B Partial central diabetes insipidus
• C Nephrogenic diabetes insipidus ✓
• D Primary polydipsia

Explanation

In nephrogenic diabetes insipidus the kidney cannot respond to vasopressin, so endogenous ADH is appropriately high given the raised osmolality, urine remains dilute despite maximal dehydration, and exogenous desmopressin produces less than a 50 percent rise in urine osmolality. In complete central DI, ADH would be undetectable and desmopressin would concentrate urine markedly. In primary polydipsia, urine concentrates normally once plasma osmolality rises.

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

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Written and medically reviewed by the StethoPrep medical team.