Which single laboratory finding best distinguishes Central Diabetes Insipidus (CDI) from Nephrogenic Diabetes Insipidus (NDI)?

Explanation

## Distinguishing CDI from NDI: The Desmopressin Challenge Test ### Pathophysiology Overview **Central Diabetes Insipidus (CDI):** - Deficiency of ADH (vasopressin) production or release from posterior pituitary - Kidneys are structurally and functionally normal - Respond appropriately to exogenous ADH (desmopressin) **Nephrogenic Diabetes Insipidus (NDI):** - Normal or elevated ADH production - Kidneys are resistant to ADH action (V~2~ receptor dysfunction or aquaporin-2 defects) - Do NOT respond to exogenous ADH ### Comparison Table | Feature | CDI | NDI | | --- | --- | --- | | **Baseline Urine Osmolality** | < 300 mOsm/kg | < 300 mOsm/kg | | **Baseline Serum ADH** | Low or absent | High or normal | | **After DDAVP (desmopressin)** | ↑ Urine osmolality > 600 mOsm/kg | No change; remains < 300 mOsm/kg | | **Serum Sodium** | Elevated (if inadequate fluid intake) | Elevated (if inadequate fluid intake) | | **Polyuria** | Present | Present | ### Key Point: **The desmopressin (DDAVP) challenge test is the gold standard discriminator.** A brisk rise in urine osmolality (> 600 mOsm/kg) after DDAVP administration confirms CDI; absence of response confirms NDI. ### High-Yield: Both conditions present with: - Polyuria (urine output > 3 L/day) - Polydipsia - Dilute urine (osmolality < 300 mOsm/kg) - Hypernatremia (if fluid intake is restricted) The **ONLY way to tell them apart** is the response to exogenous ADH. ### Clinical Pearl: In clinical practice, a simple bedside test: give desmopressin (intranasal or subcutaneous) and measure urine osmolality 1–2 hours later. If it rises significantly → CDI. If it stays low → NDI. ### Mnemonic: **"CDI responds, NDI resists"** — Central DI responds to DDAVP; Nephrogenic DI resists (is refractory to) DDAVP.