How to Confirm Methanol Poisoning
Begin fomepizole treatment immediately upon suspicion of methanol ingestion based on clinical presentation and laboratory findings—do not wait for serum methanol levels, as the diagnosis requires integrating anion gap metabolic acidosis, elevated osmolar gap, and characteristic symptoms like visual disturbances. 1
Diagnostic Criteria
Laboratory Confirmation
Calculate the anion gap using the formula: Na⁺ - (Cl⁻ + HCO₃⁻), with an elevated anion gap (particularly >27 mmol/L indicating severe toxicity) being a key diagnostic criterion. 2
Calculate the osmolar gap using: 2[measured Na (mEq/L)] + glucose (mg/dL)/18, with an elevated osmolar gap (>10-50 mOsm/kg depending on clinical context) supporting the diagnosis. 2
Clinical Presentation to Assess
Visual disturbances ranging from blurred vision to complete blindness are pathognomonic for methanol toxicity and should trigger immediate treatment. 1, 3
Ataxia may occur as a neurological manifestation. 2
Metabolic acidosis with increased anion gap is the hallmark finding, caused by accumulation of formic acid. 2, 3
Nausea, vomiting, abdominal pain, and mild CNS depression typically occur initially, followed by a latent period of 12-24 hours before severe acidosis develops. 3
Oxalate crystals in urine may be present and support toxic alcohol ingestion. 1
Differential Diagnosis Considerations
Methanol toxicity must be distinguished from other causes of high anion gap metabolic acidosis, including: 2
- Lactic acidosis
- Diabetic ketoacidosis
- Ethylene glycol poisoning
- Salicylate toxicity
- Chronic renal failure
Measure blood lactate, serum salicylate, and consider ethylene glycol levels when the diagnosis is uncertain. 2
Critical Timing Issues
The interval from ingestion to treatment is more important than the initial biochemical status or serum methanol level for determining outcome. 4, 5 This is because:
- Methanol concentrations diminish in blood as they are metabolized to toxic metabolites (formaldehyde and formic acid). 1
- By the time patients present with severe acidosis, irreversible damage may have already occurred. 6
- There is little correlation between blood methanol level and visual outcome—timing of treatment matters most. 5
When to Initiate Treatment Without Waiting for Confirmation
Start fomepizole immediately if ANY of the following are present: 1, 5
- Patient history of methanol ingestion
- Anion gap metabolic acidosis with increased osmolar gap
- Visual disturbances
- Oxalate crystals in urine
- Known ingestion exceeding 30 mL of absolute methanol
- Documented serum methanol concentration >20 mg/dL
Serum Methanol Level Interpretation
- >20 mg/dL: Initiate treatment 1
- >50 mg/dL: Indicates need for hemodialysis in addition to fomepizole 1, 5
- Undetectable or <20 mg/dL with normal pH and asymptomatic patient: Treatment may be discontinued 1
Common Diagnostic Pitfalls
Do not wait for serum methanol levels to return before starting treatment—the diagnosis is clinical and based on metabolic derangements. 1
Do not be falsely reassured by low or normal methanol levels—they may have already been metabolized to toxic formic acid. 1, 6
Do not overlook the 12-24 hour latent period—patients may initially appear well before severe toxicity develops. 6, 3
Do not forget to assess for co-ingestion of ethanol, which delays methanol metabolism and can create a longer latent period. 6, 3