What is the osmolar gap and why does it matter?

The osmolar gap is the difference between measured and calculated osmolality. An elevated gap suggests unmeasured osmoles such as ethylene glycol, methanol, or mannitol.

When should I be concerned about an elevated osmolar gap?

A gap above 10–15 mOsm/kg is often considered elevated. Gaps above 20 strongly suggest significant unmeasured osmoles and warrant urgent investigation, especially with a history of toxin exposure.

Clinical decision support

Osmolality calculator

Calculate serum osmolality and osmolar gap for dogs and cats. Review sodium, glucose, and BUN contributions, interpretation cues, and suggested next clinical checks.

Sodium (mmol/L) Glucose

Glucose unit BUN

BUN unit Measured osmolality (mOsm/kg, optional)

Educational support only. Verify with clinical judgment.

Reference ranges & conversion factors

Glucose conversion factor BUN conversion factor

Normal osmolality min (mOsm/kg) Normal osmolality max (mOsm/kg)

Osmolar gap borderline (mOsm/kg) Osmolar gap high (mOsm/kg)

Defaults are commonly cited references. Adjust to match your laboratory or institutional protocol.

Calculated osmolality

— mOsm/kg

Enter values

Enter sodium and optional glucose, BUN, and measured osmolality to calculate.

Osmolar gap —

Enter measured osmolality to enable.

Interpretation —

—

Sodium contribution —

—

Gap status —

—

Clinical next steps

Review sodium, glucose, and BUN contributions to identify the dominant driver.
If osmolar gap is elevated, consider ethylene glycol, methanol, mannitol, or other unmeasured osmoles.
Correlate with hydration status, urine output, and neurological signs.
Repeat osmolality if the clinical picture does not match the calculated value.

Safety checks

This tool supports calculation and education only.
It does not diagnose, prescribe, or replace current clinical references.
Verify patient-specific decisions with current formularies, hospital protocols, and veterinarian judgment.
Confirm glucose and BUN units match your selections.
Osmolar gap interpretation varies by laboratory and clinical context.

Basis and limits

Calculation basis: calculated osmolality = 2×Na + glucose/18 + BUN/2.8 (when glucose and BUN are in mg/dL). SI conversions are applied when mmol/L is selected.
Scope: this tool supports arithmetic. It does not diagnose specific toxins or dictate therapy.
Before use: verify units and compare results with your laboratory reference ranges.
Osmolar gap: measured osmolality minus calculated osmolality. Normal is typically <10–15 mOsm/kg.

Formula audit trail

Calculation steps will appear here for verification.

Osmolality calculator

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