A 48-year-old man with treatment-resistant bipolar disorder has been on lithium monotherapy for 6 months. He reports increased thirst and polyuria (passing 4–5 L urine daily). Serum lithium level is 0.9 mEq/L (therapeutic). Which investigation is most specific for confirming lithium-induced nephrogenic diabetes insipidus?

## Diagnosis of Lithium-Induced Nephrogenic Diabetes Insipidus ### Clinical Presentation This patient exhibits **polyuria (4–5 L/day) and polydipsia** despite a therapeutic lithium level, which is the classic triad of **nephrogenic diabetes insipidus (NDI)**. The key diagnostic challenge is distinguishing NDI from central DI and confirming the osmotic defect. ### Why Fasting Serum and Urine Osmolality is the Gold Standard **Key Point:** Nephrogenic DI is characterized by **inability of the kidney to concentrate urine in response to ADH**. The diagnostic hallmark is: - **Elevated serum osmolality** (>295 mOsm/kg) due to free water loss - **Low urine osmolality** (<300 mOsm/kg) despite high serum osmolality - This osmotic gradient (serum osmolality >> urine osmolality) is **pathognomonic** for NDI. **High-Yield:** In lithium-induced NDI: - Serum osmolality: ↑ (typically 295–310 mOsm/kg) - Urine osmolality: ↓ (typically 100–200 mOsm/kg) - Plasma ADH: ↑ (normal or elevated, because the kidney is **resistant** to ADH, not deficient in it) This **dissociation** (high ADH + low urine osmolality) is the key to diagnosis. ### Diagnostic Algorithm for Polyuria ```mermaid flowchart TD A[Polyuria + Polydipsia]:::outcome --> B[Measure fasting serum osmolality]:::action B --> C{Serum osmolality}:::decision C -->|Normal| D[Psychogenic polydipsia]:::outcome C -->|Elevated| E[Measure urine osmolality]:::action E --> F{Urine osmolality}:::decision F -->|High| G[Central DI]:::outcome F -->|Low| H[Nephrogenic DI]:::outcome H --> I{Lithium exposure?}:::decision I -->|Yes| J[Lithium-induced NDI]:::outcome I -->|No| K[Idiopathic or genetic NDI]:::outcome ``` ### Comparison of DI Subtypes | Feature | Central DI | Nephrogenic DI | |---|---|---| | **Serum osmolality** | ↑ (>295) | ↑ (>295) | | **Urine osmolality** | ↓ (<300) | ↓ (<300) | | **Plasma ADH** | ↓ (low) | ↑ (normal or high) | | **Response to desmopressin** | Urine osmolality ↑ | No response | | **Lithium association** | No | Yes (20–40% of users) | **Clinical Pearl:** The **desmopressin (DDAVP) challenge test** can differentiate central from nephrogenic DI: in central DI, urine osmolality rises >50% after DDAVP; in NDI, there is minimal or no response. However, **fasting serum and urine osmolality** are the **initial, most specific investigations** and do not require pharmacological challenge. ### Mechanism of Lithium-Induced NDI 1. **Acute phase (reversible):** Lithium inhibits adenylyl cyclase → ↓ cAMP → impaired aquaporin-2 (AQP2) channel insertion in collecting duct 2. **Chronic phase (partially reversible):** Chronic lithium exposure causes: - Downregulation of AQP2 expression - Chronic interstitial fibrosis - Permanent loss of concentrating ability **Tip:** NDI may persist even after lithium discontinuation if chronic fibrosis has occurred. Early detection via osmolality testing allows timely intervention (amiloride, NSAIDs, adequate hydration).