During a water deprivation test, a patient fails to concentrate urine (peak osmolality 210 mOsm/kg) after adequate dehydration. After subcutaneous desmopressin, urine osmolality rises to 480 mOsm/kg, an increase of more than 50 percent. What is the diagnosis?

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

Explanation

C large rise in urine osmolality, typically above 50 percent, after exogenous desmopressin shows intact renal responsiveness to vasopressin, establishing central diabetes insipidus. In nephrogenic diabetes insipidus the kidney cannot respond, so desmopressin raises osmolality by less than about 45 percent. Primary polydipsia concentrates urine during the deprivation phase itself. SIADH is incompatible with dilute polyuric urine and would present with hyponatraemia rather than this picture.

Reference: Williams Textbook of Endocrinology, 14th ed.

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