A 52-year-old woman with bipolar disorder has been on lithium 900 mg daily for 3 years with good mood stabilization. She presents with coarse tremor, confusion, and ataxia. Serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). Which investigation is most appropriate to assess the extent of lithium-induced organ damage?

## Lithium Toxicity and Chronic Organ Damage Assessment ### Clinical Context This patient presents with **acute lithium toxicity** (tremor, confusion, ataxia with supratherapeutic level of 2.8 mEq/L). However, chronic lithium use carries the risk of **nephrogenic diabetes insipidus (NDI)** and **chronic kidney disease (CKD)**, which must be monitored systematically. ### Why Renal Function Assessment is the Investigation of Choice **Key Point:** Lithium causes dose-dependent, often irreversible renal damage through chronic interstitial nephritis and nephrogenic DI. Serum creatinine and 24-hour urine creatinine clearance (or eGFR) are the **gold-standard investigations** for assessing glomerular filtration rate and detecting CKD in lithium-treated patients. **High-Yield:** Lithium nephrotoxicity occurs in 20–40% of long-term users and is one of the most serious chronic adverse effects. Baseline renal function should be documented, and monitoring should occur every 6–12 months during therapy. ### Monitoring Protocol for Lithium Patients | Investigation | Baseline | Frequency | Purpose | |---|---|---|---| | Serum creatinine / eGFR | Yes | Every 6–12 months | Detect declining GFR | | 24-hour urine creatinine clearance | Yes | Annually or if creatinine rises | Assess GFR trend | | Thyroid function (TSH, free T4) | Yes | Every 6–12 months | Screen for hypothyroidism | | Serum lithium level | Yes | 5–7 days after initiation, then every 3–6 months | Maintain therapeutic range | | Urine osmolality (fasting) | Optional | If polyuria suspected | Assess for NDI | **Clinical Pearl:** In this patient with 3 years of lithium exposure and acute toxicity, renal function assessment is **urgent** to determine if there is underlying CKD that may have contributed to lithium accumulation and toxicity. ### Mechanism of Lithium Nephrotoxicity 1. Acute phase: Reversible polyuria via ADH-resistant NDI (lithium blocks aquaporin-2 channels) 2. Chronic phase: Progressive interstitial fibrosis, glomerulosclerosis, and irreversible renal impairment 3. Risk factors: High doses, prolonged duration (>5 years), dehydration, concurrent NSAIDs **Tip:** Always counsel patients on adequate hydration and avoid NSAIDs; ACE inhibitors may offer some renoprotection.