How is serum osmolality calculated?

Calculated osmolality = 2 × Na + (Glucose / 18) + (BUN / 2.8). This estimates the contribution of the three major solutes. If ethanol is suspected, add ethanol (mg/dL) / 4.6 to the calculation.

What is the osmolar gap?

Osmolar gap = Measured osmolality − Calculated osmolality. A normal osmolar gap is 10) suggests the presence of unmeasured osmoles such as methanol, ethylene glycol, isopropyl alcohol, or mannitol — toxicologic emergencies.

What is normal serum osmolality?

Normal serum osmolality is 275–295 mOsm/kg. Values above 295 indicate hyperosmolality (seen in hypernatremia, hyperglycemia, or toxic ingestions). Values below 275 indicate hypo-osmolality (typically associated with hyponatremia). Osmolality < 320 is maintained by intact thirst and ADH mechanisms.

When should you measure vs. calculate osmolality?

Calculate osmolality to screen for osmolar gap (compare with measured value). Measure osmolality directly when toxic alcohol ingestion is suspected, to evaluate hyponatremia etiology, and when calculated values seem discordant with clinical picture. The osmolar gap is only useful when measured osmolality is available.

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Serum Osmolality Calculator

Calculate serum osmolality and osmolar gap. Enter measured osmolality to compute the osmolar gap for toxic alcohol evaluation.

Eingaben

Sodium (mEq/L)

Glucose (mg/dL)

BUN (mg/dL)

Ethanol (mg/dL) (optional)

Measured Osmolality (mOsm/kg) (optional, for gap)

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Enter Na, glucose, and BUN.

Normal Osmolality275–295 mOsm/kg

Normal Osmolar Gap< 10 mOsm/kg

Elevated GapConsider toxic alcohols

Formula: Calc Osm = 2 × Na + Glucose/18 + BUN/2.8 + Ethanol/4.6

Haftungsausschluss: Nur zu Bildungszwecken. Kein Ersatz für klinisches Urteil.

Anwendung

Calculate serum osmolality in patients with altered mental status, suspected toxic alcohol ingestion, hyponatremia workup, or unexplained metabolic acidosis. The osmolar gap helps detect unmeasured osmoles (methanol, ethylene glycol, isopropanol).

Clinical pearls

Osmolar gap > 10 mOsm/kg raises concern for toxic alcohols — methanol, ethylene glycol, isopropanol.
A normal osmolar gap does NOT rule out toxic alcohol ingestion if the parent compound has been metabolized to its toxic acid metabolites.
Ethanol contributes significantly to osmolality (≈ 22 mOsm/kg per 100 mg/dL) — always account for it.
Chronic kidney disease and lactic acidosis can also cause mild osmolar gap elevation.

References

Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659.
Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208-225.

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Über dieses Tool

What Is Serum Osmolality?

Serum osmolality reflects the concentration of dissolved solutes in the blood, measured in milliosmoles per kilogram of water (mOsm/kg). The primary contributors are sodium (and its associated anions), glucose, and urea (BUN). Normal serum osmolality ranges from 275 to 295 mOsm/kg. Calculated osmolality estimates this value from measured lab values using the formula: Calc Osm = 2 × Na + Glucose/18 + BUN/2.8. When ethanol is present, Ethanol/4.6 is added.

The Osmolar Gap

The osmolar gap is the difference between the measured (by freezing point depression) and calculated osmolality. A normal osmolar gap is less than 10 mOsm/kg. An elevated osmolar gap indicates the presence of unmeasured osmotically active substances in the blood. The classic clinical concern is toxic alcohol ingestion — methanol, ethylene glycol, and isopropanol all increase the osmolar gap because they are osmotically active but not accounted for in the standard osmolality formula.

Clinical Application

In a patient presenting with unexplained metabolic acidosis, altered mental status, or suspected ingestion, calculating the osmolar gap is a critical step. Methanol and ethylene glycol ingestion classically produce both an elevated anion gap metabolic acidosis AND an elevated osmolar gap. However, as these parent alcohols are metabolized to their toxic acid metabolites (formic acid and oxalic/glycolic acid, respectively), the osmolar gap normalizes while the anion gap increases — the so-called "osmolar gap-anion gap" transition. Isopropanol increases the osmolar gap without causing metabolic acidosis (it is metabolized to acetone, not an acid).

🔑 Klinische Hinweise

Elevated osmolar gap + elevated anion gap = think methanol or ethylene glycol (early presentation).
Elevated osmolar gap + normal anion gap = think isopropanol (metabolized to acetone, not an acid).
A normal osmolar gap does NOT exclude toxic alcohol ingestion if presentation is delayed and parent compound is metabolized.
Baseline osmolar gap varies between individuals (−2 to +10) — a "normal" gap of 9 could represent a significant change from a patient's baseline of 0.

Schlüsselreferenzen

Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208-225.
Purssell RA, et al. The osmolal gap. Ann Emerg Med. 2001;38(6):653-659.
Hoffman RS, et al. Osmol gaps revisited: normal values and limitations. J Toxicol Clin Toxicol. 1993;31(1):81-93.

Formel zuletzt überprüft: Februar 2026