A 45-year-old woman on ICU has a severe traumatic brain injury. Her ICP has been controlled at 18 mmHg. On day 5, she develops new polyuria (5 L/day)...

Question

A 45-year-old woman on ICU has a severe traumatic brain injury. Her ICP has been controlled at 18 mmHg. On day 5, she develops new polyuria (>5 L/day), hypernatraemia (Na+ 155 mmol/L), and plasma osmolality 320 mOsm/kg. Urine osmolality is 120 mOsm/kg (dilute). What test can differentiate central from nephrogenic diabetes insipidus?

Accepted Answer

IV/SC desmopressin (DDAVP) test - administer desmopressin and measure urine osmolality response; in central DI, urine osmolality increases >50% after DDAVP (ADH-responsive); in nephrogenic DI, urine osmolality does not increase significantly (ADH-resistant)

Answer

Fluid deprivation test - unsafe in this setting

Answer

MRI brain only

Answer

Serum cortisol

Answer

No test can differentiate them

uk-fficm MCQ (SBA): Central vs Nephrogenic DI