In central diabetes insipidus, what labs characterize the condition and how does desmopressin testing help?

• A. High urine osmolality with low serum osmolality
• B. Low urine osmolality (<300 mOsm/kg) with high serum osmolality
• C. High serum osmolality with low ADH
• D. Normal urine osmolality and normal serum osmolality

Answer: (B)

Central diabetes insipidus reflects a deficiency of ADH, so the kidney can’t concentrate urine. The body loses a lot of free water, which makes the urine very dilute and allows serum osmolality to rise. So the characteristic labs are a dilute urine with low osmolality (typically <300 mOsm/kg) and an elevated serum osmolality from the water loss. Desmopressin testing clarifies the problem: giving desmopressin, a synthetic ADH, should prompt the kidneys to reabsorb water and concentrate the urine in central DI, causing a significant rise in urine osmolality and a drop in urine volume. If there were no response to desmopressin, that would point toward nephrogenic DI, where the kidney doesn’t respond to ADH.