A 48-year-old woman on lithium 600 mg twice daily for bipolar I disorder presents with coarse tremor, confusion, ataxia, and diarrhea for 2 days. Her serum creatinine is 1.8 mg/dL (baseline 0.9 mg/dL). Which investigation is most appropriate to confirm the diagnosis and assess severity of lithium toxicity?

## Acute Lithium Toxicity: Diagnostic Confirmation **Key Point:** Acute lithium toxicity is confirmed by measuring **serum lithium level** (drawn at a standardized time, ideally 12 hours post-dose or immediately). Clinical signs alone are insufficient; serum level is mandatory to confirm toxicity and guide management. ### Clinical Presentation of Acute Lithium Toxicity The patient's triad of **neurotoxicity** (tremor, confusion, ataxia) + **gastrointestinal symptoms** (diarrhea) + **acute renal dysfunction** (creatinine rise from 0.9 to 1.8 mg/dL) is highly suggestive of acute lithium toxicity. However, diagnosis must be confirmed by serum lithium measurement. ### Why Serum Lithium Level? 1. **Diagnostic gold standard:** Serum lithium > 1.5 mEq/L is associated with toxicity; levels > 2.0 mEq/L indicate severe toxicity. 2. **Severity stratification:** The serum level correlates with clinical severity and guides treatment intensity (supportive care vs. hemodialysis). 3. **Immediate action:** Knowing the serum level allows rapid decision-making about whether hemodialysis is indicated. 4. **Differential diagnosis:** Confirms lithium as the cause (vs. other causes of acute confusion and renal failure). ### Why Other Investigations Are Not First-Line | Investigation | Why Not Appropriate Here | | --- | --- | | **EEG** | Non-specific; may show slowing but does not confirm lithium toxicity or guide acute management. | | **Urine/serum osmolality** | Relevant for nephrogenic diabetes insipidus (chronic lithium effect), not acute toxicity. | | **TSH** | Relevant for chronic lithium-induced hypothyroidism, not acute toxicity. | **High-Yield:** In acute lithium toxicity, **serum lithium level is the single most important investigation** because it confirms the diagnosis and determines whether hemodialysis is needed (typically indicated if level > 2.0–2.5 mEq/L or if severe clinical features persist despite supportive care). **Clinical Pearl:** Acute renal dysfunction (as seen in this patient) can paradoxically worsen lithium toxicity by reducing renal clearance, creating a vicious cycle. Serum level measurement is critical to detect this and initiate hemodialysis promptly. **Mnemonic:** **SALT = Serum And Lithium Toxicity** — Always measure serum lithium level first in suspected toxicity.