A 42-year-old man on lithium 750 mg daily for bipolar I disorder presents with tremor, confusion, ataxia, and coarse tremor. He had vomiting and diarrhea 2 days ago. Which investigation is most appropriate as the first step to evaluate suspected lithium toxicity?

Explanation

## Diagnosis of Acute Lithium Toxicity ### Clinical Presentation The patient presents with classic signs of lithium toxicity: tremor, confusion, ataxia, and coarse tremor (vs. fine tremor seen with therapeutic use). Gastrointestinal loss (vomiting, diarrhea) increases lithium reabsorption in the proximal tubule, raising serum levels. ### Why Serum Lithium Level First? **Key Point:** Serum lithium level is the ONLY specific, quantitative test that confirms the diagnosis of lithium toxicity and guides management decisions. **High-Yield:** - **Therapeutic range:** 0.6–1.2 mEq/L - **Mild toxicity:** 1.5–2.0 mEq/L (tremor, nausea, diarrhea) - **Moderate toxicity:** 2.0–3.5 mEq/L (confusion, ataxia, coarse tremor) - **Severe toxicity:** >3.5 mEq/L (seizures, cardiac arrhythmias, renal failure, death) **Mnemonic:** **LITE** — **L**ithium level, **I**ntake history, **T**herapeutic drug monitoring, **E**lectrolytes & renal function (supportive tests). ### Diagnostic Hierarchy ```mermaid flowchart TD A[Suspected Lithium Toxicity]:::outcome --> B[Serum Lithium Level]:::action B --> C{Level & Severity?}:::decision C -->|Mild: 1.5-2.0| D[Supportive care, hydration]:::action C -->|Moderate: 2.0-3.5| E[Hemodialysis consideration]:::action C -->|Severe: >3.5| F[Urgent hemodialysis]:::urgent B --> G[Renal function & Electrolytes]:::action B --> H[EEG/CT if atypical features]:::action ``` ### Why Not the Other Options? **Electroencephalography (EEG)** — Non-specific. EEG changes (slowing, theta waves) may occur in toxicity but do not quantify severity or guide acute management. It is a supportive test, not diagnostic. **Renal function tests and serum electrolytes** — Essential supportive investigations (lithium is renally cleared; hypernatremia/hypokalemia worsen toxicity) but do NOT confirm toxicity. Must be done alongside lithium level, not instead of it. **Computed tomography (CT) brain** — Indicated only if focal neurological signs, seizures, or atypical presentation suggest alternative diagnosis (stroke, hemorrhage). Not the first-line investigation for lithium toxicity. **Clinical Pearl:** In acute toxicity with GI losses, lithium reabsorption increases because the proximal tubule interprets volume depletion as a signal to reabsorb all filtered solutes, including lithium. This is why aggressive IV saline rehydration is part of management.