Osmolal Gap
Glucose Units
Urea / BUN Units
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The osmolal gap is the numerical difference between the measured serum osmolality (obtained by laboratory freezing-point depression osmometry) and the calculated osmolality derived from the major measured solutes: sodium, glucose, and urea/BUN. In a healthy individual, these three solutes account for virtually all plasma osmolality, so the gap should be small (≤10 mOsm/kg). When additional unmeasured osmotically active substances are present — most critically the toxic alcohols methanol, ethylene glycol, and isopropanol — the measured osmolality rises while the calculated value remains unchanged, creating a detectable gap. The osmolal gap is therefore a screening tool for toxic alcohol ingestion and is an essential component of the toxicological and metabolic workup. An elevated osmolal gap (>10 mOsm/kg) in the context of altered mental status, metabolic acidosis with elevated anion gap, or a history suggesting toxic alcohol ingestion represents a medical emergency requiring immediate management including consideration of fomepizole (an alcohol dehydrogenase inhibitor), haemodialysis, and antidotal therapy. Importantly, the osmolal gap alone cannot exclude toxic alcohol poisoning — in late presentations, the toxic alcohol may have already been metabolised to its acid metabolites (formate from methanol, glycolate and oxalate from ethylene glycol), reducing the parent alcohol concentration and thereby normalising the osmolal gap while the anion gap remains elevated.
Calculated Posm = 2×Na + Glucose/18 + BUN/2.8 (mg/dL units); SI: 2×Na + Glucose + Urea (mmol/L). Osmolal Gap = Measured osmolality − Calculated osmolality
- 1Obtain a measured serum osmolality from the laboratory using freezing-point depression osmometry — this measures all osmotically active particles regardless of identity.
- 2Collect the serum chemistry values needed for the formula: sodium (Na, mEq/L), glucose (mg/dL or mmol/L), and BUN/urea (mg/dL or mmol/L).
- 3Calculate the predicted osmolality using: 2×Na + Glucose/18 + BUN/2.8 (for mg/dL units). For SI units: 2×Na + Glucose(mmol/L) + Urea(mmol/L). The factor of 2 for sodium accounts for chloride and bicarbonate as accompanying anions. Dividing glucose by 18 and BUN by 2.8 converts mg/dL to mmol/L (molecular weight adjustments).
- 4Subtract calculated from measured: Osmolal Gap = Measured − Calculated.
- 5Interpret: Normal gap ≤10 mOsm/kg. Gap >10 mOsm/kg suggests unmeasured osmoles.
- 6In the clinical context of elevated anion gap metabolic acidosis plus elevated osmolal gap, strongly consider toxic alcohol ingestion — calculate which alcohol could account for the gap (e.g., ethanol gap = osmolal gap × 4.6 mg/dL per mOsm).
- 7Recognise the temporal evolution: early toxic alcohol poisoning shows elevated osmolal gap AND elevated anion gap; late presentation shows normalised osmolal gap (alcohol metabolised) but persistent elevated anion gap from acid metabolites.
Osmolal gap of 44 is critically elevated — methanol or ethylene glycol ingestion must be excluded emergently; administer fomepizole while awaiting toxic alcohol levels
Calculated = 2×140 + 108/18 + 14/2.8 = 280 + 6 + 5 = 291 mOsm/kg. Gap = 335 − 291 = 44. An osmolal gap of 44 corresponds to approximately 44 × 3.2 mg/dL ≈ 141 mg/dL methanol — a potentially lethal level.
Elevated gap likely due to ethanol — each 46 mg/dL of ethanol raises osmolality by approximately 10 mOsm/kg. Estimated blood ethanol ≈ 143 mg/dL
Calculated = 2×138 + 90/18 + 10/2.8 = 276 + 5 + 3.6 = 284.6 ≈ 285. Gap = 316 − 285 = 31. Estimated ethanol = 31 × 4.6 ≈ 143 mg/dL. Ethanol is the commonest cause of elevated osmolal gap and must be considered before attributing the gap to toxic alcohols.
Normal/negative gap — toxic alcohols unlikely to be present in significant quantity at this time point
Calculated = 2×140 + 180/18 + 20/2.8 = 280 + 10 + 7.1 = 297. Gap = 292 − 297 = −5. Negative values occur from lab variation or measurement differences; values between −10 and +10 are considered normal. This patient's acidosis has another cause (e.g., DKA, lactic acidosis).
Despite near-normal osmolal gap, the elevated anion gap (28) from formate accumulation should raise suspicion for late methanol toxicity — do not be falsely reassured by a normalising osmolal gap
As methanol is metabolised to formate over 12–24 hours, the parent alcohol (which contributes to the osmolal gap) disappears while the acid metabolite (formate) accumulates and drives up the anion gap. A patient with unexplained high anion gap acidosis and history of alcohol use requires toxic alcohol levels even if osmolal gap is normal.
Emergency toxicology screening for suspected methanol or ethylene glycol ingestion in patients with altered mental status and metabolic acidosis, representing an important application area for the Osmolal Gap in professional and analytical contexts where accurate osmolal gap calculations directly support informed decision-making, strategic planning, and performance optimization
Estimating blood ethanol concentration when formal alcohol levels are unavailable or delayed, representing an important application area for the Osmolal Gap in professional and analytical contexts where accurate osmolal gap calculations directly support informed decision-making, strategic planning, and performance optimization
Monitoring mannitol accumulation in neurocritical care to guide repeat dosing and avoid toxicity, representing an important application area for the Osmolal Gap in professional and analytical contexts where accurate osmolal gap calculations directly support informed decision-making, strategic planning, and performance optimization
Detecting propylene glycol toxicity in ICU patients on prolonged benzodiazepine infusions, representing an important application area for the Osmolal Gap in professional and analytical contexts where accurate osmolal gap calculations directly support informed decision-making, strategic planning, and performance optimization
Differentiating causes of elevated anion gap metabolic acidosis by pairing osmolal gap with anion gap delta-delta analysis, representing an important application area for the Osmolal Gap in professional and analytical contexts where accurate osmolal gap calculations directly support informed decision-making, strategic planning, and performance optimization
Late-presenting toxic alcohol poisoning
{'title': 'Late-presenting toxic alcohol poisoning', 'body': 'Patients who present many hours after methanol or ethylene glycol ingestion may have a near-normal osmolal gap if the parent alcohol has been metabolised. The key clue is a severe unexplained anion gap metabolic acidosis — formate (from methanol) or glycolate/oxalate (from ethylene glycol) drives the anion gap while the osmolal gap normalises. Always send toxic alcohol levels in any patient with unexplained high anion gap acidosis and possible toxic alcohol exposure.'}
Mannitol administration in neurocritical care
{'title': 'Mannitol administration in neurocritical care', 'body': 'Bolus mannitol infusion for raised intracranial pressure causes a predictable and large osmolal gap. A gap of 20–40 mOsm/kg in a neurosurgical patient on mannitol is expected and does not indicate toxic alcohol poisoning. Serum mannitol levels are rarely measured; the osmolal gap can be used to monitor mannitol accumulation and guide repeat dosing.'}
Severe diabetic ketoacidosis
{'title': 'Severe diabetic ketoacidosis', 'body': 'Acetone and beta-hydroxybutyrate (in very high concentrations during severe DKA) can contribute modest elevations to the osmolal gap, typically 5–15 mOsm/kg. This rarely causes diagnostic confusion but should be considered when interpreting a mildly elevated osmolal gap in a known diabetic patient with DKA. The anion gap in DKA is driven primarily by ketoacids, not the osmolal gap contribution.'}
Propylene glycol toxicity in the ICU
{'title': 'Propylene glycol toxicity in the ICU', 'body': 'Prolonged IV lorazepam infusion (>72 hours at high doses) can cause propylene glycol accumulation with osmolal gap elevation, lactic acidosis (D-lactic acid from propylene glycol metabolism), and nephrotoxicity. Switching to midazolam or dexmedetomidine, or reducing lorazepam dose, resolves the toxicity. An unexplained osmolal gap elevation in an ICU patient on sustained benzodiazepine infusion should trigger propylene glycol measurement.'}
Hyperlipidaemia and pseudohyponatraemia
{'title': 'Hyperlipidaemia and pseudohyponatraemia', 'body': 'In severe hypertriglyceridaemia (TG >1000 mg/dL), the measured sodium may be falsely low (pseudohyponatraemia) while measured osmolality remains appropriate. The osmolal gap may appear falsely elevated or falsely normal depending on how sodium was measured. Direct-reading potentiometric (ISE undiluted) sodium measurement and lipemic plasma inspection help identify this artefact.'}
| Substance | Molecular Weight (g/mol) | mOsm/kg per mg/dL | mg/dL per 10 mOsm/kg gap |
|---|---|---|---|
| Ethanol | 46 | 0.22 | 46 |
| Methanol | 32 | 0.31 | 32 |
| Ethylene glycol | 62 | 0.16 | 62 |
| Isopropanol | 60 | 0.17 | 60 |
| Mannitol | 182 | 0.055 | 182 |
| Propylene glycol | 76 | 0.13 | 76 |
What is a normal osmolal gap?
The normal osmolal gap is generally accepted as ≤10 mOsm/kg, though reference values vary slightly by laboratory (range −2 to +10 in most studies). The normal range assumes the standard formula accounts for all major osmoles. Values up to 10 can occur from analytical variability between the osmometer and chemistry analyser. A gap >10 is abnormal and requires explanation.
What are the causes of an elevated osmolal gap?
The major causes are: toxic alcohols (ethanol, methanol, ethylene glycol, isopropanol — most dangerous); mannitol administration (common in neurosurgical patients); contrast agents containing iodine; severe diabetic ketoacidosis (from acetone and beta-hydroxybutyrate); glycerol; propylene glycol (found in IV formulations of lorazepam, diazepam, phenobarbitone); severe hypertriglyceridaemia; and hyperproteinaemia (pseudohypo-osmolality). Ethanol is the commonest cause in emergency practice.
Can a normal osmolal gap exclude toxic alcohol poisoning?
No — this is a critical clinical point. A normal osmolal gap does NOT exclude toxic alcohol poisoning, particularly in late presentations. Methanol and ethylene glycol are metabolised within 12–24 hours of ingestion. As the parent alcohol is eliminated, the osmolal gap normalises, but the toxic acid metabolites (formate from methanol; glycolate and oxalate from ethylene glycol) accumulate and drive a severe elevated anion gap metabolic acidosis. A patient with unexplained anion gap acidosis requires toxic alcohol levels regardless of osmolal gap.
How does ethanol affect the osmolal gap?
Ethanol is an osmotically active molecule that directly raises measured osmolality without being included in the standard Posm formula. Each 46 mg/dL of ethanol (molecular weight 46 g/mol) raises osmolality by approximately 10 mOsm/kg. Some institutions modify the formula to include ethanol: Posm = 2×Na + Glucose/18 + BUN/2.8 + Ethanol/4.6, which reduces the osmolal gap in known ethanol ingestion and allows detection of additional unmeasured osmoles.
What is pseudohyponatraemia and how does it affect osmolality?
In severe hypertriglyceridaemia or hyperproteinaemia, the aqueous fraction of plasma (where sodium is dissolved) is reduced by the excess lipid or protein. Flame photometry and some indirect ISE methods measure sodium in the total plasma volume rather than the aqueous phase, reporting falsely low sodium (pseudohyponatraemia). Measured osmolality (which reflects aqueous phase) remains normal or appropriate, creating a normal or negative osmolal gap that may help identify the artefact.
What is the osmolal gap contribution of mannitol?
Mannitol (molecular weight 182 g/mol) is commonly infused in neurosurgery, traumatic brain injury, and acute angle-closure glaucoma. Each gram/dL of mannitol raises osmolality by approximately 5.5 mOsm/kg. Patients receiving mannitol infusions will have elevated osmolal gaps that do not indicate toxic alcohol poisoning. Mannitol levels or knowledge of recent infusion history are needed to interpret the gap correctly in these patients.
Is the osmolal gap the same as the osmotic gap?
Yes — osmolal gap and osmotic gap are used interchangeably in clinical practice, referring to the same calculation. The term 'osmolal' (relating to osmolality, measured in mOsm/kg water) is technically more precise than 'osmotic' in this context, though both are encountered in the literature and are clinically equivalent. This is particularly important in the context of osmolal gap calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise osmolal gap computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.
What is propylene glycol toxicity and how does it affect osmolal gap?
Propylene glycol is used as a solubilising agent in IV formulations of lorazepam, diazepam, phenobarbitone, and etomidate. In patients receiving prolonged high-dose IV lorazepam infusions (common in ICU sedation), propylene glycol can accumulate, causing an elevated osmolal gap, anion gap metabolic acidosis, and renal toxicity. This is an important iatrogenic cause of osmolal gap elevation in ICU patients and should be considered alongside toxic alcohols.
Pro Tip
When methanol or ethylene glycol poisoning is suspected, do not wait for toxic alcohol levels to return before starting fomepizole. The osmolal gap can be used to estimate the toxic alcohol concentration: estimated alcohol (mg/dL) = osmolal gap × molecular weight / 10. For methanol, gap × 3.2 ≈ methanol mg/dL. A serum methanol >20 mg/dL is an indication for fomepizole; >50 mg/dL typically warrants urgent haemodialysis.
Did you know?
The term 'osmolal gap' was introduced in the 1960s as osmometry became more widely available. Ironically, one of the major contributors to osmolal gap research was Erwin Smolens and colleagues who noticed that inebriated patients in the emergency department had measured osmolalities far higher than their basic chemistry explained — leading to the systematic use of osmolal gap in toxicology screening.
References
- ›Kruse JA, Cadnapaphornchai P — The serum osmole gap (J Crit Care 1994)
- ›Jammalamadaka D, Raissi S — Toxic alcohol ingestion: ethylene glycol, methanol and isopropanol (Prim Care 2010)
- ›Purssell RA et al. — Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap (Ann Emerg Med 2001)
- ›TOXBASE — Methanol poisoning management
- ›Marx et al. — Rosen's Emergency Medicine, 9th ed. — Toxic Alcohols chapter