Osmol Gap
Calculates the difference between measured and calculated serum osmolality to detect unmeasured osmoles (e.g. toxic alcohols, mannitol)
Score interpretation
Osmol gap ≤ 10 mOsm/kg: No significant unmeasured osmoles.
→ Toxic alcohols (methanol, ethylene glycol, isopropanol) unlikely. Consider other causes of metabolic acidosis.
Osmol gap 11–20 mOsm/kg: Borderline elevation.
→ Clinical correlation required. Consider ethanol, mannitol, propylene glycol, renal failure. Low-grade toxic alcohol ingestion possible.
Osmol gap > 20 mOsm/kg: Significant unmeasured osmoles present.
→ Toxic alcohol ingestion (methanol/ethylene glycol) must be excluded urgently. Consider fomepizole empirically. Contact Poisons Information. Send urgent toxic alcohol levels.
Interpretation bands for the Osmol Gap. Apply clinical judgement and local guidance.
References
- Purssell RA, et al. The use of the osmole gap in diagnosing toxic ingestion. J Toxicol Clin Toxicol. 2004;42(3):261–272.
- Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011;58(3):480–484.
Related
Curated clinical cross-links plus same-class fallbacks.
- Sodium Bicarbonate · Alkali Therapy (Metabolic Acidosis / CKD-Related Acidosis)
- Trimetazidine · Metabolic Anti-Anginal
- Liquid paraffin with white soft paraffin and wool alcohols · Emollient
- L-Carnitine · Antidote / Metabolic Agent
- Macrogol (Polyethylene Glycol) · Osmotic Laxative
- Macrogol (Polyethylene Glycol) · Osmotic Laxative
- Difficult Airway Algorithm (DAS) · DAS 2015; Royal College of Anaesthetists
- Major Haemorrhage Protocol · NICE NG24; UK MHP guidelines
- New-Onset Atrial Fibrillation · ESC 2020 AF Guidelines; NICE NG196
- Hypertensive Emergency · ESC/ESH 2018 Hypertension Guidelines; NICE NG136
- Bradycardia Management · Resuscitation Council UK ABCDE; ESC 2021 Pacing Guidelines
- Ventricular Tachycardia / Fibrillation · Resuscitation Council UK ACLS; ESC 2022 Ventricular Arrhythmia Guidelines
Featured in these MRCEM clinical pathways
The Osmol Gap is covered in detail — with RCEM/NICE evidence base, indications and pitfalls — in the following exam-focused pathways on our sister siteReviseMRCEM.
MRCEM Primary / Intermediate / OSCE candidates: each pathway includes exam-style questions, RCEM/NICE citations, and FAQ summaries.
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.