Central vs Nephrogenic DI DDAVP Test — FRCA Final MCQ

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ModerateNeuroanaesthesiaCentral vs Nephrogenic DI DDAVP TestFRCA Final

A 55-year-old man is on the ICU after a traumatic brain injury. He develops polyuria (6 L/day) with dilute urine (osmolality 120 mOsm/kg). Serum sodium is 152 mmol/L. A trial of desmopressin (DDAVP) concentrates his urine to 600 mOsm/kg. This confirms:

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Correct answer: E — Nephrogenic diabetes insipidus

The response to DDAVP differentiates central from nephrogenic DI: Central DI (lack of ADH production from posterior pituitary) responds to exogenous DDAVP – urine concentrates (>600 mOsm/kg). Nephrogenic DI (renal resistance to ADH) does NOT respond to DDAVP. This patient's urine concentrated with DDAVP, confirming central DI – consistent with TBI causing hypothalamic-pituitary damage. Treatment: regular DDAVP (intranasal, oral, or SC) titrated to urine output, plus free water replacement.

Reference: NICE – 2017 – IV fluid therapy; RCOA – 2023 – Neuroanaesthesia: post-TBI DI