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Printed on 7/19/2026
For informational purposes only. This is not medical advice.
The osmolar gap is the difference between measured serum osmolality and calculated serum osmolality. An elevated gap can indicate unmeasured osmotically active substances and is often used in toxicology and critical care workups.
Formula: Osmolar Gap = Measured Osmolality - Calculated Osmolality.
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Input the measured serum osmolality from the laboratory (normal approximately 285–295 mOsm/kg). This is measured directly by freezing-point depression. Do NOT use the calculated osmolality from your metabolic panel — use only the actual measured value from a dedicated osmolality test ordered specifically.
The calculator uses the Weisberg formula: Calculated Osmolality = 2×Na + BUN/2.8 + Glucose/18. Enter serum sodium (mEq/L), BUN (mg/dL), and glucose (mg/dL) from a simultaneous blood draw. If ethanol level is available, enter it as well — ethanol contributes to osmolality and an ethanol-corrected osmolar gap is more specific for toxic alcohols.
Osmolar Gap = Measured − Calculated. Normal gap is below 10 mOsm/kg (some references use <15). A gap above 10–15 suggests unmeasured osmotically active substances. Toxic alcohols (methanol, ethylene glycol, isopropanol) combined with high anion gap acidosis is the critical diagnosis to rule out. Always consider ethanol, mannitol (ICU patients), propylene glycol (from IV lorazepam or vancomycin), and severe illness. A normal gap does NOT rule out toxic alcohols — the gap normalizes as alcohols are metabolized.
Emergency physicians, toxicologists
Methanol (from windshield washer fluid, moonshine) and ethylene glycol (antifreeze) both cause an early osmolar gap before they are metabolized to toxic organic acids. An elevated osmolar gap combined with high anion gap metabolic acidosis is the classic presentation. Immediate treatment (fomepizole, ethanol, or hemodialysis) depends on early recognition — the osmolar gap is a critical screening tool. Contact Poison Control (1-800-222-1222) immediately for any suspected toxic alcohol ingestion.
Emergency physicians, hospitalists, ICU teams
When a patient has unexplained high anion gap acidosis and the standard causes (MUDPILES: Methanol, Uremia, DKA, Propylene glycol, INH, Lactic acidosis, Ethylene glycol, Salicylates) are being worked up, the osmolar gap helps specifically screen for toxic alcohols. Combined high AG + osmolar gap = toxic alcohol until proven otherwise. Even if one gap is normal, the other may still be elevated depending on timing.
Emergency physicians, toxicologists
Isopropanol (rubbing alcohol) causes an elevated osmolar gap WITHOUT anion gap acidosis — because isopropanol is metabolized to acetone (a ketone), not to an organic acid. The clinical fingerprint is: osmolar gap elevated, anion gap normal, serum ketones elevated, no significant acidosis. This is an important distinction from methanol and ethylene glycol. Isopropanol poisoning is generally less lethal than methanol or ethylene glycol.
Neurointensivists, ICU nurses, neurosurgeons
Mannitol (an osmotic agent used to treat cerebral edema and elevated ICP) is an unmeasured osmole that elevates the measured osmolality and widens the osmolar gap. Monitoring serial osmolar gaps in mannitol-treated patients prevents dangerous osmolality accumulation above 320 mOsm/kg. When the osmolar gap exceeds 20–25 in a mannitol-treated patient, the next dose should be withheld and osmolality rechecked.
Intensivists, clinical pharmacists
Propylene glycol is the solvent used in IV formulations of lorazepam, diazepam, and vancomycin. In ICU patients receiving high-dose or prolonged infusions of these agents, propylene glycol accumulates and causes an elevated osmolar gap — sometimes with lactic acidosis. This iatrogenic cause is often overlooked. Calculate daily propylene glycol load and monitor osmolar gap in patients on high-dose IV lorazepam infusions.
If the patient has consumed ethanol, it contributes approximately 4.6 mOsm per 10 mg/dL of serum ethanol level (or ethanol level / 4.6 = contribution in mOsm/kg). To get the ethanol-corrected osmolar gap — which is more specific for toxic alcohols other than ethanol — subtract the ethanol contribution from the measured osmolality before calculating the gap. A large gap that is entirely explained by ethanol is less concerning for other toxic alcohols than a gap that persists even after accounting for ethanol.
In methanol and ethylene glycol poisoning, the classic progression is: early — osmolar gap elevated, anion gap normal (parent alcohol present, not yet metabolized). Middle — both gaps elevated. Late — osmolar gap normalized, anion gap markedly elevated (parent alcohol fully metabolized to formic acid from methanol, or oxalic acid from ethylene glycol). At late presentation, the osmolar gap may be normal despite severe poisoning. Test both gaps and interpret together.
Isopropanol is the exception among toxic alcohols. It metabolizes to acetone — a ketone, not an organic acid — so there is no acidosis. The clinical triad: elevated osmolar gap + normal anion gap + elevated serum ketones + no significant acidosis. If a patient smells of acetone, has ketonemia without acidosis, and has no hyperglycemia to explain DKA, isopropanol should be high on the differential.
This is the most critical clinical pearl: as methanol or ethylene glycol is fully metabolized to organic acids, the parent alcohol (which contributes to osmolality) is consumed. The osmolar gap narrows and may normalize while the anion gap continues to widen. A patient presenting 12–24 hours after methanol ingestion may have a normal osmolar gap but a dangerously elevated anion gap acidosis. Never use a normal osmolar gap alone to rule out methanol or ethylene glycol if clinical suspicion is high.
Ethylene glycol is metabolized to oxalic acid, which combines with calcium to form calcium oxalate crystals deposited in the renal tubules. Envelope-shaped (monohydrate) or octahedral (dihydrate) calcium oxalate crystals in a urinalysis — especially in a patient with altered mental status, high anion gap acidosis, and elevated osmolar gap — is pathognomonic for ethylene glycol poisoning. This finding should prompt immediate fomepizole administration and nephrology consultation for potential hemodialysis.
Calculate the estimated propylene glycol load in ICU patients. IV lorazepam contains propylene glycol as a carrier — a patient receiving 10 mg/hr continuously can accumulate significant levels. Propylene glycol causes osmolar gap elevation and, with high levels, lactic acidosis. If an ICU patient's osmolar gap is rising without obvious cause, review all IV medication solvents, particularly high-dose benzodiazepine infusions.
Several formulas exist for calculated osmolality. The Weisberg formula (2×Na + BUN/2.8 + Glucose/18) is the most commonly used and validated. Some older formulas include 10 or 9 as a constant term (e.g., 2×Na + BUN/2.8 + Glucose/18 + 10). Institutional variation in formula choice can affect the normal reference range for osmolar gap — know which formula your clinical lab uses when comparing to your calculator result.
US Poison Control (1-800-222-1222) provides 24/7 guidance for toxic alcohol management. Call early — before all laboratory results are back. Fomepizole (4-methylpyrazole) is the antidote for both methanol and ethylene glycol poisoning; it inhibits alcohol dehydrogenase and prevents formation of toxic metabolites. Start fomepizole empirically if clinical suspicion is high, even before confirmation, because early treatment dramatically improves outcomes.
Lactic acidosis, DKA, uremia, and other critical illness states can cause a modestly elevated osmolar gap (typically 10–20 mOsm/kg) through accumulation of organic acids and other small molecules. This 'baseline illness gap' can complicate interpretation in the ICU. A gap below 20 in a critically ill patient without specific toxic exposure may represent illness-related osmoles rather than toxic alcohols. Clinical context and targeted toxicology testing are essential.
Osmolar gap interpretation is reviewed in Hoffman et al., Goldfrank's Toxicologic Emergencies (11th ed., 2019). The Weisberg formula for calculated osmolality (2×Na + BUN/2.8 + Glucose/18) is the most widely validated. Sensitivity for toxic alcohols ranges from 47–90% depending on timing of presentation; gap normalizes as alcohol is metabolized to organic acids (Kraut & Kurtz, CJASN 2008).
A larger positive gap suggests unmeasured osmoles and should prompt focused toxicology and metabolic evaluation in the appropriate clinical scenario.
Use this tool when measured and calculated osmolality are both available and there is concern for toxic ingestion, unexplained acidosis, or altered mental status.
Gap thresholds vary by assay and institution. Osmolar gap is a screening clue, not a definitive diagnosis, and may be normal in late toxic alcohol presentations.
For related assessments, see Serum Osmolality, Anion Gap and ABG Interpreter.
Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.
April 21, 2026 · trust-baseline
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Calculate serum osmolality from sodium, glucose, and BUN. Normal range: 275–295 mOsm/kg. Osmolal gap >10 suggests toxic alcohol ingestion (methanol, ethylene glycol, isopropanol) or other unmeasured osmoles.
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