A 35-year-old man is brought to the emergency department with altered mental status, visual disturbances, and metabolic acidosis (pH 7.18, bicarbonate 12 mEq/L). He consumed an unknown clear liquid 8 hours ago. Clinical suspicion is high for methanol poisoning. Which investigation is most specific for confirming methanol toxicity?

Explanation

## Diagnosis of Methanol Poisoning ### Why Serum/Urine Methanol Level is Gold Standard **Key Point:** Direct measurement of methanol concentration by gas chromatography or high-performance liquid chromatography (HPLC) is the definitive and most specific confirmatory test for methanol poisoning. **High-Yield:** Methanol levels >20 mg/dL are associated with significant toxicity; levels >50 mg/dL warrant aggressive treatment including fomepizole or ethanol. ### Role of Other Investigations | Investigation | Purpose | Limitation | |---|---|---| | Osmolar gap | Screens for toxic alcohols (non-specific) | Elevated in any toxic alcohol poisoning; not diagnostic for methanol | | ABG analysis | Assesses severity of metabolic acidosis | Indicates severity, not specific cause | | Fundoscopy | Clinical sign of methanol toxicity | Late finding; not confirmatory; absent in early stages | ### Clinical Pearl **Clinical Pearl:** The osmolar gap (measured osmolality − calculated osmolality) is elevated early in methanol poisoning but normalizes as methanol is metabolized to formic acid. Therefore, a normal osmolar gap does NOT exclude methanol poisoning, especially if the patient presents late. ### Pathophysiology Context Methanol is metabolized by alcohol dehydrogenase to formaldehyde, then by aldehyde dehydrogenase to formic acid (formate). Formic acid causes the severe metabolic acidosis and visual toxicity (optic nerve damage). Direct measurement of methanol in serum or urine confirms the diagnosis before complications develop. **Mnemonic:** **METH** = **M**ethanol level by **E**nzyme-based **T**esting or **H**PLC/GC (the only specific test). [cite:Park 26e Ch 13]