During a desmopressin stimulation test, central DI shows what change?

• A. No change in urine osmolality
• B. Increase in urine osmolality and decrease in urine volume
• C. Decrease in urine osmolality
• D. Immediate hyponatremia

Answer: (B)

Desmopressin acts on receptors in the kidney collecting ducts to increase water reabsorption. In central diabetes insipidus there is a deficiency of vasopressin, so urine is usually very dilute and produced in large volumes. When desmopressin is given, the kidneys can respond to this ADH analogue, concentrating the urine. This shows up as a rise in urine osmolality and a fall in urine volume. This diagnostic response helps distinguish central DI from nephrogenic DI, where the kidneys do not respond to ADH and there is little or no change in osmolality or volume after desmopressin. Hyponatremia is not the expected immediate result of the test; it could occur only if there is excessive water retention from over-treatment, not as the typical initial effect.