What is the normal osmolal gap?

A normal osmolal gap is less than 10 mOsm/kg. Some sources use a threshold of 10–15 mOsm/kg depending on the laboratory method used to measure osmolality. Values consistently above 10 mOsm/kg warrant further investigation.

What causes an elevated osmolal gap?

Common causes include toxic alcohol ingestion (ethanol, methanol, ethylene glycol, isopropanol), mannitol administration, severe hyperproteinemia or hyperlipidemia (pseudohyponatremia), propylene glycol toxicity, and severe renal failure with accumulation of unmeasured solutes.

Can ethanol cause an elevated osmolal gap?

Yes. Ethanol is a small, osmotically active molecule. Each 100 mg/dL of ethanol adds roughly 22 mOsm/kg to measured osmolality. If a blood alcohol level is known, ethanol's contribution (BAL/4.6) can be subtracted from the gap to reveal a residual gap from other toxins.

Can the osmolal gap be normal with toxic alcohol ingestion?

Yes — a normal gap does not rule out toxic alcohol ingestion if enough time has passed for the parent alcohol to be metabolized. Methanol and ethylene glycol are converted to toxic acids; later in the course the anion gap rises while the osmolal gap falls. Both gaps must be interpreted together.

Is this calculator a substitute for clinical evaluation?

No. This tool is for educational and reference use only. Suspected toxic alcohol ingestion is a medical emergency requiring immediate clinical assessment, blood gas analysis, toxicology consultation, and treatment initiation. Do not delay care to use this calculator.

Serum Osmolal Gap Calculator

Name: Serum Osmolal Gap Calculator
Availability: InStock
Author: Nham Vu

Enter Na, glucose, BUN, and measured osmolality to calculate the osmolal gap and screen for unmeasured osmoles including toxic alcohols.

Serum Values

Standard US units (mg/dL for glucose and BUN)

Sodium (Na⁺) mEq/L — normal 135–145

Glucose mg/dL — normal 70–100

BUN (Blood Urea Nitrogen) mg/dL — normal 7–20

Measured Osmolality mOsm/kg — normal 275–295

Results

Enter values and click Calculate

Quick Reference

Osmolal Gap Ranges

Gap (mOsm/kg)	Interpretation
< 10	Normal — no significant unmeasured osmoles
10 – 20	Mildly elevated — investigate clinical context
20 – 50	Elevated — toxic alcohol or mannitol likely
> 50	Markedly elevated — significant toxin burden

Common Causes of Elevated Gap

Substance	Clinical context
Ethanol	Most common; adds ~22 mOsm per 100 mg/dL
Methanol	Early poisoning; later gap falls, anion gap rises
Ethylene glycol	Antifreeze; renal failure, calcium oxalate crystals
Isopropanol	Rubbing alcohol; ketonemia without acidosis
Mannitol	Therapeutic use in raised ICP
Propylene glycol	Prolonged IV lorazepam or other infusions

For educational and reference use only. Suspected toxic alcohol ingestion is a medical emergency — seek immediate clinical evaluation and do not rely on this calculator for treatment decisions.

Summary

Enter Na, glucose, BUN, and measured osmolality to calculate the osmolal gap and screen for unmeasured osmoles including toxic alcohols.

How it works

Enter serum sodium (Na) in mEq/L, glucose in mg/dL, and BUN in mg/dL.
Enter the measured serum osmolality in mOsm/kg from the laboratory report.
Calculated osmolality = 2×Na + (glucose/18) + (BUN/2.8).
Osmolal gap = measured osmolality − calculated osmolality.
A gap below 10 mOsm/kg is considered normal; above 10 suggests unmeasured osmoles.
Review the interpretation panel for clinical guidance and differential diagnosis.

Use cases

Screening for toxic alcohol ingestion (methanol, ethylene glycol, isopropanol) in the emergency department.
Evaluating unexplained altered mental status with high anion gap metabolic acidosis.
Assessing propylene glycol toxicity in patients on prolonged lorazepam or other infusions.
Estimating ethanol contribution to osmolality when a blood alcohol level is available.
Medical education on the relationship between osmolality and serum solutes.
ICU monitoring of patients receiving mannitol therapy.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu