Test Description

The Toxic Alcohol Family

The clinically significant toxic alcohols are:

  • Methanol: Found in windshield washer fluid, fuel additives, solvents, moonshine. Metabolized to formaldehyde then formic acid (causes blindness and death).
  • Ethylene glycol: Found in antifreeze, coolants, brake fluid. Metabolized to glycolaldehyde, glycolic acid, and oxalic acid (causes renal failure and death).
  • Isopropanol: Rubbing alcohol. Metabolized to acetone. Least dangerous — does not cause anion gap acidosis (see ethanol level page).

Why These Are Dangerous

The parent compounds (methanol, ethylene glycol) are relatively non-toxic themselves. The toxicity comes from their metabolites, which are produced by the enzyme alcohol dehydrogenase (ADH):

  • Methanol → Formaldehyde → Formic acid (causes optic nerve and CNS toxicity)
  • Ethylene glycol → Glycolaldehyde → Glycolic acid → Oxalic acid (causes acute kidney injury via calcium oxalate crystal deposition)

The Osmol Gap and Anion Gap Relationship

This is the critical concept for toxic alcohol diagnosis:

  • Early (parent compound present): Elevated osmol gap + Normal anion gap — the unmeasured osmoles from the parent alcohol raise the osmol gap
  • Intermediate: Elevated osmol gap + Elevated anion gap — both parent compound and metabolites coexist
  • Late (fully metabolized): Normal osmol gap + Elevated anion gap — parent compound has been metabolized to toxic acids
A normal osmol gap does NOT rule out toxic alcohol poisoning. If the patient presents late, the parent compound may be fully metabolized (osmol gap is normal) but the toxic metabolites have already accumulated (anion gap is elevated). Always assess both gaps.
Quick Reference
  • Normal Level: Undetectable (methanol and ethylene glycol should not be present)
  • Toxic Level: Any detectable level in a symptomatic patient
  • Fomepizole Threshold: >20 mg/dL (methanol or ethylene glycol)
  • Osmol Gap: Elevated (>10 mOsm/kg) early — before parent compound is metabolized
  • Anion Gap: Elevated later — as toxic metabolites (formic acid, glycolic acid) accumulate
  • Antidote: Fomepizole (4-MP) — inhibits alcohol dehydrogenase; prevents toxic metabolite formation
  • Hemodialysis: For severe poisoning — removes parent compound AND toxic metabolites
  • Key Point: Osmol gap and anion gap evolve in opposite directions over time — early = high osmol gap; late = high anion gap
Calculating Osmol Gap

Calculated vs. Measured Osmolality

Calculated Osmolality = 2(Na) + Glucose/18 + BUN/2.8 + EtOH/4.6

Osmol Gap = Measured Osmolality - Calculated Osmolality

  • Normal osmol gap: <10 mOsm/kg
  • Suggestive of toxic alcohol: >10 mOsm/kg (after accounting for ethanol)
  • Highly suspicious: >25 mOsm/kg

Converting Osmol Gap to Estimated Toxic Alcohol Level

  • Methanol: Level (mg/dL) = Osmol Gap × 3.2
  • Ethylene glycol: Level (mg/dL) = Osmol Gap × 6.2
Lab osmolality must be measured by freezing point depression, not vapor pressure osmometry. Vapor pressure methods underestimate osmolality in the presence of volatile alcohols (methanol, ethanol) and will give a falsely low osmol gap.
Methanol vs. Ethylene Glycol
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Feature Methanol Ethylene Glycol
Sources Windshield washer fluid, solvents, fuel additives Antifreeze, coolants, brake fluid
Toxic metabolite Formic acid Glycolic acid, oxalic acid
Organ toxicity Optic nerve (blindness), CNS (basal ganglia necrosis) Kidneys (AKI from calcium oxalate crystals)
Classic findings Visual complaints ("snowfield vision"), blindness, afferent pupillary defect Flank pain, hematuria, calcium oxalate crystalluria, QTc prolongation (from hypocalcemia)
Urine crystals None Calcium oxalate (envelope or needle-shaped)
Wood lamp Negative May fluoresce (from fluorescein additive in some antifreeze)
Lethal dose ~1 mL/kg (or ~30 mL in adults) ~1.5 mL/kg (or ~100 mL in adults)
Adjunct therapy Folic acid (enhances formic acid metabolism) Thiamine + Pyridoxine (shunt glycolate away from oxalate)
Treatment

Fomepizole (4-Methylpyrazole, Antizol)

Preferred antidote — competitive inhibitor of alcohol dehydrogenase:

  • Loading dose: 15 mg/kg IV
  • Maintenance: 10 mg/kg IV every 12 hours for 4 doses
  • Then: 15 mg/kg IV every 12 hours until levels undetectable and patient improving
  • During hemodialysis: Dose every 4 hours (fomepizole is dialyzable)

Ethanol (Alternative If Fomepizole Unavailable)

  • Target ethanol level: 100-150 mg/dL (ethanol competes for ADH)
  • More difficult to titrate; causes intoxication; preferred only if fomepizole is unavailable

Indications for Hemodialysis

  • Severe metabolic acidosis (pH <7.25-7.30)
  • Renal failure
  • Visual symptoms (methanol)
  • Toxic alcohol level >50 mg/dL
  • Significant end-organ damage
  • Worsening clinical status despite treatment

Adjunctive Therapies

  • Methanol poisoning: Folic acid (folate) 1 mg/kg IV (max 50 mg) every 4-6 hours — enhances metabolism of formic acid to CO2 + H2O
  • Ethylene glycol poisoning: Thiamine 100 mg IV + Pyridoxine (B6) 50 mg IV every 6 hours — diverts glycolate metabolism away from oxalate
  • Sodium bicarbonate: For severe acidosis; also helps keep formic acid/glycolic acid in ionized form (less CNS penetration)
Do not wait for confirmatory levels to start treatment. Specific toxic alcohol levels may take hours to days to return from reference labs. If clinical suspicion is high (elevated osmol gap, anion gap metabolic acidosis, compatible history), start fomepizole immediately and arrange hemodialysis.
Clinical Pearls
"The gaps evolve over time": Think of it as a seesaw — early poisoning shows elevated osmol gap (parent compound) with normal anion gap. As the parent compound is metabolized, the osmol gap falls and the anion gap rises. A patient presenting late may have a normal osmol gap with a massive anion gap.
Co-ingestion of ethanol is protective: If a patient co-ingests ethanol with methanol or ethylene glycol, the ethanol preferentially occupies ADH and slows the formation of toxic metabolites. This buys time but does not eliminate the need for fomepizole and dialysis.
Methanol blindness can be permanent. Formic acid causes direct retinal and optic nerve toxicity. Visual symptoms (blurred vision, "snowfield" vision, decreased visual acuity, photophobia) are an ominous sign and warrant emergent hemodialysis regardless of level.
Check calcium in ethylene glycol poisoning: Oxalic acid binds calcium to form calcium oxalate crystals. This can cause significant hypocalcemia, leading to QTc prolongation, arrhythmias, and tetany. Monitor calcium and ECG closely.
Wood lamp of urine: Some antifreeze products contain fluorescein dye, which causes urine to fluoresce under Wood lamp. However, this is unreliable — many modern antifreeze products no longer contain fluorescein. A negative test does NOT rule out ethylene glycol ingestion.
Propylene glycol is not ethylene glycol: Propylene glycol (used in IV lorazepam, some food products) can cause an osmol gap and lactic acidosis but is far less toxic. Drug history (especially high-dose IV lorazepam or phenobarbital infusions) can help differentiate.
References
  1. Kraut, J. A., & Kurtz, I. (2008). Toxic alcohol ingestions: clinical features, diagnosis, and management. Clinical Journal of the American Society of Nephrology, 3(1), 208-225.
  2. Brent, J. (2009). Fomepizole for ethylene glycol and methanol poisoning. New England Journal of Medicine, 360(21), 2216-2223.
  3. Roberts, D. M., et al. (2015). Recommendations for the role of extracorporeal treatments in the management of acute methanol poisoning (EXTRIP). Critical Care Medicine, 43(2), 461-472.
  4. Farkas, Josh MD. (2015). Toxic alcohols. EMCrit Project — Internet Book of Critical Care.
  5. Nickson, C. (n.d.). Toxic alcohols. Life in the Fast Lane (LITFL).
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