What are the initial tests to order for suspected methanol intoxication?

Initial Tests for Suspected Methanol Intoxication

For suspected methanol intoxication, the initial diagnostic tests should include arterial blood gases with pH, serum electrolytes with anion gap calculation, serum osmolality for osmolar gap, and direct measurement of methanol and formate levels.

Core Laboratory Tests

First-line Tests

• Arterial blood gases

  • Metabolic acidosis (pH < 7.35) is a hallmark finding
  • Severe acidosis (pH < 7.20) correlates with high short-term mortality 1

• Serum electrolytes

  • Calculate anion gap: [Na+] - ([Cl-] + [HCO3-])
  • High anion gap (> 12 mEq/L) suggests methanol poisoning
  • Anion gap of 23-27 mmol/L indicates moderate to severe toxicity 2

• Serum osmolality (measured) and calculation of osmolar gap

  • Osmolar gap = Measured osmolality - Calculated osmolality
  • Calculated osmolality = 2[Na+] + [Glucose]/18 + [BUN]/2.8 + [Ethanol]/4.6
  • Osmolar gap > 10 mOsm/kg suggests presence of toxic alcohols

Specific Diagnostic Tests

• Serum methanol concentration

  • Gold standard for diagnosis
  • Levels > 20 mg/dL warrant treatment 3
  • Levels ≥ 50 mg/dL indicate need for hemodialysis 3

• Serum formate concentration

  • Direct measurement of methanol's toxic metabolite
  • More clinically relevant than methanol levels for predicting toxicity
  • Levels ≥ 3.7 mmol/L associated with visual toxicity 4
  • Levels ≥ 11-12 mmol/L associated with visual/CNS sequelae and mortality 4
  • Bedside formate test strips may provide rapid diagnosis 5

Additional Important Tests

• Lactate level

  • Elevated in methanol poisoning due to cellular hypoxia
  • Levels ≥ 7.0 mmol/L associated with poor outcomes 4
  • Lactate ≥ 10 mmol/L may indicate concomitant cyanide poisoning if exposure was from a fire 1

• Blood alcohol (ethanol) level

  • Important to determine if co-ingestion occurred
  • Presence of ethanol may delay metabolism of methanol to toxic formate

• Renal function tests

  • BUN and creatinine to assess kidney function
  • Renal failure may develop and affect treatment decisions

• Liver function tests

  • Transaminases may be elevated with repeated fomepizole dosing 3

• Complete blood count

  • Monitor for eosinophilia which can occur with fomepizole treatment 3

Imaging and Additional Considerations

• Ophthalmologic examination

  • Visual disturbances are characteristic of methanol poisoning
  • Fundoscopic exam may reveal optic disc hyperemia or papilledema

• Toxicology screening

  • If intentional ingestion is suspected, screen for other toxic co-ingestions 1
  • 44% of intentional poisonings involve co-ingestion of other substances 1

Clinical Pearls and Pitfalls

• Timing considerations: Symptoms may appear hours to days after exposure, reflecting time needed for toxic metabolite accumulation 6

• Diagnostic challenges: Normal anion gap and osmolar gap can occur depending on time since exposure and co-ingestion of ethanol 6

• Treatment initiation: Begin fomepizole treatment immediately upon suspicion of methanol ingestion, even before laboratory confirmation 3

• Monitoring: Frequent measurements of blood gases, pH, electrolytes, and methanol/formate levels are essential throughout treatment 3

• Hemodialysis indications: Consider hemodialysis for methanol levels ≥ 50 mg/dL, significant metabolic acidosis, or renal failure 3

Remember that early diagnosis and treatment are critical to prevent mortality and long-term sequelae such as blindness and neurological damage from methanol poisoning.