Toxic Alcohol Level from Osmolal Gap
Toxic Alcohol Level from Osmolal Gap
Estimate a methanol, ethylene glycol, isopropanol, acetone or propylene glycol concentration from the osmolal gap while a direct assay is awaited.
Estimated concentration
2 inputs → mg/dLOsmolal gap 25 mOsm/kg, methanol suspected
Formula and factors
- molecular weight ÷ 10
- one tenth of the molecular weight converts mOsm/kg back into mg/dL — methanol 32.04 gives 3.204, ethylene glycol 62.07 gives 6.207, isopropanol 60.10 gives 6.010, acetone 58.08 gives 5.808, propylene glycol 76.10 gives 7.610
- osmolal gap
- measured osmolality minus calculated osmolality, with any measured ethanol already subtracted in the calculation
- assumption
- the entire gap is attributed to the single selected substance, which is what makes this an estimate rather than a measurement
Worked example
Osmolal gap 25 mOsm/kg, methanol suspected
Methanol factor = 32.04 ÷ 10 = 3.204
25 × 3.204 = 80.1 mg/dL estimated methanol
Above 50 mg/dL → antidote plus consideration of extracorporeal removal, pending a direct assay
Molecular weights and conversion factors
| Substance | Molecular weight | Factor (MW ÷ 10) |
|---|---|---|
| Methanol | 32.04 | 3.204 |
| Ethylene glycol | 62.07 | 6.207 |
| Isopropanol | 60.10 | 6.010 |
| Acetone | 58.08 | 5.808 |
| Propylene glycol | 76.10 | 7.610 |
What the estimated concentration means by substance
| Substance | Toxic metabolite | Management |
|---|---|---|
| Methanol | Formic acid | Antidote at 20 mg/dL; visual loss and severe acidosis; dialysis considered |
| Ethylene glycol | Glycolic and oxalic acid | Antidote at 20 mg/dL; acute kidney injury and calcium oxalate crystals; dialysis considered |
| Isopropanol | Acetone — not an acid | Supportive; ketosis without acidosis |
| Acetone | None | Supportive |
| Propylene glycol | Lactate | Usually iatrogenic from drug diluents; stop the infusion |
What the estimate can and cannot tell you
When a toxic alcohol is suspected and no direct assay is available within a useful timeframe, the osmolal gap can be converted into an approximate concentration. The arithmetic is simple: each milliosmole of unmeasured solute corresponds to one tenth of a molecular weight in mg/dL, so the gap is multiplied by 3.204 for methanol, 6.207 for ethylene glycol, 6.010 for isopropanol, 5.808 for acetone and 7.610 for propylene glycol. The result is a bridge to treatment while the definitive assay is awaited, not a substitute for it.
The banding differs by substance for a reason. The 20 mg/dL antidote threshold and the 50 mg/dL threshold at which extracorporeal removal is usually considered apply to methanol and ethylene glycol only, because those two are metabolised by alcohol dehydrogenase into formic acid and into glycolic and oxalic acid respectively. Isopropanol and acetone cause sedation and ketosis but no acidosis, and are managed supportively; propylene glycol accumulation is usually iatrogenic, from the diluent in intravenous lorazepam and some other drugs, and resolves when the infusion stops.
The estimate rests on one large assumption: that the entire osmolal gap comes from the single substance selected. That fails whenever ethanol is co-ingested and has not been measured and subtracted, which is common, and it fails whenever the gap has a different cause altogether — ketoacidosis, lactic acidosis, renal failure or hypertriglyceridaemia. It also fails in the opposite direction late in a poisoning, when the parent alcohol has largely been converted to its acid and the gap has closed while the patient deteriorates.
Treat the number as a prompt to act rather than a measurement to rely on. A direct methanol or ethylene glycol assay is the definitive test, and where it is available quickly it should govern. Toxic alcohol poisoning is time-critical, and antidote and dialysis decisions should be guided by a poisons centre or clinical toxicology service; this calculator supports that conversation and never replaces it.
Frequently asked questions
How do you estimate a methanol level from the osmolal gap?
Multiply the osmolal gap by 3.204, one tenth of methanol's molecular weight of 32.04. An osmolal gap of 25 mOsm/kg gives an estimated methanol of about 80 mg/dL.
Do the 20 and 50 mg/dL thresholds apply to every substance here?
No. They apply to methanol and ethylene glycol only, because those two produce toxic acid metabolites. Isopropanol and acetone are managed supportively, and propylene glycol accumulation is usually iatrogenic from drug diluents.
What makes this estimate unreliable?
It assumes the whole osmolal gap comes from one substance. Unmeasured co-ingested ethanol, and other causes of a raised gap such as ketoacidosis, lactic acidosis, renal failure or hypertriglyceridaemia, all inflate the estimate.
Should I wait for a direct assay before treating?
No. A direct assay is the definitive test, but where it is not rapidly available the estimate is used to start an alcohol dehydrogenase inhibitor. Poisoning management is time-critical and should be guided by a poisons centre or clinical toxicology service, which this tool supports rather than replaces.
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References
- Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208–225.
- Barceloux DG, Bond GR, Krenzelok EP, et al. American Academy of Clinical Toxicology practice guidelines on the treatment of methanol poisoning. J Toxicol Clin Toxicol. 2002;40(4):415–446.
- Barceloux DG, Krenzelok EP, Olson K, Watson W. American Academy of Clinical Toxicology practice guidelines on the treatment of ethylene glycol poisoning. J Toxicol Clin Toxicol. 1999;37(5):537–560.