Serum Osmolality & Osmolal Gap Calculator
Calculated serum osmolality and osmolal gap help evaluate hyponatremia, hypernatremia, and detect toxic alcohol ingestion (methanol, ethylene glycol, isopropyl alcohol).
About
Serum osmolality is a measure of the total concentration of dissolved solutes in the blood. Calculated osmolality uses the major osmotic solutes (sodium, glucose, BUN, and ethanol). The osmolal gap — the difference between measured and calculated osmolality — is a sensitive screening tool for the presence of unmeasured osmotically active substances. Normal osmolal gap is <10 mOsm/kg. An elevated gap >10 suggests the presence of toxic alcohols (methanol, ethylene glycol, isopropyl alcohol), which should be confirmed by serum levels. Low osmolality suggests overhydration or hyponatremia; high osmolality suggests dehydration, hypernatremia, or hyperglycemia.
Formula
Calculated Osm = 2 × Na + Glucose/18 + BUN/2.8 + (Ethanol/4.6 if present). Osmolal Gap = Measured − Calculated.
Calculated osmolality uses the following conversions: glucose in mg/dL divided by 18 gives mmol/L; BUN in mg/dL divided by 2.8 gives mmol/L (or divide by 6 for urea); ethanol in mg/dL divided by 4.6 gives mmol/L.
Score Interpretation
Serum osmolality measurement is critical in the evaluation of electrolyte disorders and suspected toxic alcohol ingestion. The osmolal gap is a particularly valuable emergency department screening tool for detecting methanol, ethylene glycol, and isopropyl alcohol poisoning before confirmatory serum levels are available.
Normal Osmolality — 275–295
Normal serum osmolality 275-295 mOsm/kg.
Management: No specific intervention required.
Low Osmolality — 0–274.99
Low osmolality. Evaluate hyponatremia and/or overhydration.
Management: Assess volume status and correct underlying cause.
Elevated Osmolality — 295.01+
High osmolality. Evaluate hypernatremia, hyperglycemia, or uremia.
Management: Identify and treat underlying cause.
Reference Ranges
| Population | Normal Range | Notes |
|---|---|---|
| Adults | 275-295 mOsm/kg | Osmolal gap normal <10 mOsm/kg |
Dr. Khaled Hassan
Dr. Khaled is a cardiology consultant with experience in acute cardiac care.
View medical review board & editorial policy →Example Calculation
A 45-year-old found confused: Na 140, glucose 90, BUN 14, measured osm 350. Calculated = 2×140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290. Osmolal gap = 350 − 290 = 60 (>10). Suspect toxic alcohol ingestion.
Related Conditions
Common Mistakes
Using urea instead of BUN in the formula
Use BUN (blood urea nitrogen), not urea. BUN = urea/2.14. If using urea, divide by 6 instead of 2.8.
Frequently Asked Questions
What is a normal osmolal gap?
References
- Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659. PubMed