A 42-year-old woman with a history of recurrent nephrolithiasis presents with polyuria (urine output 4–5 L/day), polydipsia, and serum sodium of 148 mEq/L. Serum osmolality is 305 mOsm/kg and urine osmolality is 150 mOsm/kg. Which investigation is most appropriate to differentiate between central diabetes insipidus (CDI) and nephrogenic diabetes insipidus (NDI)?

Explanation

## Differentiating Central from Nephrogenic Diabetes Insipidus ### Clinical Presentation The patient has diabetes insipidus (DI) characterized by: - Polyuria (> 3 L/day) - Polydipsia - Hypernatremia (Na+ 148 mEq/L) - High serum osmolality (305 mOsm/kg) - Inappropriately dilute urine (150 mOsm/kg) despite hyperosmolality The history of lithium use (for nephrolithiasis prevention, though not stated, lithium is a known cause of NDI) raises suspicion for nephrogenic DI. ### Water Deprivation Test: The Gold Standard **Key Point:** The water deprivation test (WDT) followed by desmopressin (DDAVP) administration is the definitive investigation to differentiate CDI from NDI. ### Test Protocol and Interpretation ```mermaid flowchart TD A[Water Deprivation Test]:::action --> B[Withhold water for 8-16 hours] B --> C[Measure urine osmolality] C --> D{Urine osmolality response?}:::decision D -->|Concentrates to > 600 mOsm/kg| E[Normal response - Rule out DI]:::outcome D -->|Remains dilute < 300 mOsm/kg| F[Abnormal - DI present]:::outcome F --> G[Administer DDAVP 10 mcg IM/IV]:::action G --> H{Urine osmolality after DDAVP?}:::decision H -->|Increases > 50% or > 600 mOsm/kg| I[Central DI - ADH deficiency]:::outcome H -->|No change or minimal increase| J[Nephrogenic DI - Kidney resistance]:::outcome ``` ### Interpretation Table | Stage | Finding | Interpretation | |-------|---------|----------------| | **Baseline** | Urine osmolality < 300 mOsm/kg + hypernatremia | Suggests DI | | **After water deprivation** | Urine osmolality remains < 300 mOsm/kg | Confirms DI | | **After DDAVP** | Urine osmolality ↑ > 50% (to > 600 mOsm/kg) | **Central DI** | | **After DDAVP** | Urine osmolality unchanged or ↑ < 10% | **Nephrogenic DI** | **High-Yield:** In CDI, the kidney CAN concentrate urine when exogenous ADH (DDAVP) is provided. In NDI, the kidney CANNOT concentrate urine even with DDAVP because the problem is renal resistance, not ADH deficiency. ### Why This Test Works **Clinical Pearl:** - **Central DI:** Kidneys are normal but lack ADH → water deprivation increases plasma osmolality → endogenous ADH release is absent or inadequate → urine remains dilute. DDAVP replaces missing ADH → urine concentrates. - **Nephrogenic DI:** Kidneys are resistant to ADH (genetic mutation, lithium, hypercalcemia) → even with high endogenous or exogenous ADH, urine cannot concentrate. ### Safety Considerations **Warning:** Water deprivation test must be performed under medical supervision in a hospital setting. Risk of severe hypernatremia and dehydration. Terminate test if: - Serum sodium > 150 mEq/L - Patient becomes symptomatic (confusion, seizures) - Urine osmolality plateaus despite continued deprivation [cite:Harrison 21e Ch 286]