Desmopressin testing helps differentiate central from nephrogenic diabetes insipidus based on what response?

• A. An increase in urine osmolality after desmopressin indicates central DI
• B. No change indicates central DI
• C. Decrease in serum osmolality after desmopressin indicates central DI
• D. Increase in urine sodium after desmopressin indicates central DI

Answer: (A)

Desmopressin testing hinges on how the kidneys respond to a synthetic ADH analogue. In central diabetes insipidus, there is a deficiency of ADH, so giving desmopressin provides the signal the body is missing. The collecting ducts respond by reabsorbing more water, making the urine more concentrated, which appears as a noticeable rise in urine osmolality. In contrast, nephrogenic diabetes insipidus involves the kidneys being unresponsive to ADH—no matter how much desmopressin you give, the urine remains dilute and urine osmolality doesn’t rise significantly. So the hallmark finding is a marked increase in urine osmolality after desmopressin, signaling central DI.