A 35-year-old man with a 2-week history of polyuria and polydipsia presents to the clinic. Serum sodium is 152 mEq/L and urine osmolality is 150 mOsm/kg after overnight fluid restriction. A water deprivation test is performed: urine osmolality remains 160 mOsm/kg. Which single feature best distinguishes Central Diabetes Insipidus from Nephrogenic Diabetes Insipidus in this patient?

Explanation

## Distinguishing Central from Nephrogenic Diabetes Insipidus ### Key Pathophysiology Both Central DI (CDI) and Nephrogenic DI (NDI) present with polyuria, polydipsia, and hypernatremia. The critical difference lies in **renal responsiveness to ADH**. ### Diagnostic Algorithm ```mermaid flowchart TD A[Polyuria + Hypernatremia]:::outcome --> B[Water Deprivation Test]:::action B --> C{Urine osmolality increases?}:::decision C -->|No increase| D[Administer Desmopressin]:::action C -->|Yes| E[Primary Polydipsia]:::outcome D --> F{Urine osmolality now >600?}:::decision F -->|Yes| G[Central DI]:::outcome F -->|No| H[Nephrogenic DI]:::outcome ``` ### Comparison Table | Feature | Central DI | Nephrogenic DI | | --- | --- | --- | | **ADH levels** | Low/absent | High (compensatory) | | **Renal response to ADH** | Present | Absent | | **Urine osmolality after desmopressin** | ↑ to >600 mOsm/kg | Remains <300 mOsm/kg | | **Water deprivation test** | Urine osmolality ↑ slightly | No change | | **Desmopressin challenge** | **Diagnostic** | No response | | **Causes** | Pituitary surgery, trauma, tumors | Lithium, hypercalcemia, genetic | **Key Point:** The **desmopressin (synthetic ADH) challenge test** is the gold-standard discriminator. Central DI kidneys are ADH-deficient but ADH-responsive; Nephrogenic DI kidneys are ADH-resistant. **High-Yield:** In this case, the patient failed water deprivation (urine osmolality 150→160 mOsm/kg), confirming DI. Desmopressin administration would increase urine osmolality to >600 mOsm/kg in Central DI, but remain <300 mOsm/kg in Nephrogenic DI. **Clinical Pearl:** Nephrogenic DI patients often require thiazide diuretics (paradoxically) or NSAIDs to reduce urine output, whereas Central DI responds dramatically to desmopressin replacement. **Mnemonic:** **CADI** = Central ADH Deficiency (responds to desmopressin); **NADI** = Nephrogenic ADH Insensitivity (does NOT respond).