osmolar gap calculator

What is the osmolar gap?

The osmolar gap is the difference between measured serum osmolality (from the lab) and calculated serum osmolality (from common blood chemistry values). Clinicians use it as a screening clue for unmeasured osmoles in blood, especially when toxic alcohol ingestion is a concern.

In practical terms:

• Measured osmolality comes from an osmometer.
• Calculated osmolality is estimated from sodium, glucose, blood urea nitrogen (BUN), and sometimes ethanol.
• Osmolar gap = measured osmolality − calculated osmolality.

How to use this calculator

Step-by-step

• Enter measured serum osmolality in mOsm/kg.
• Enter sodium, glucose, and BUN (required).
• Add potassium and ethanol if available.
• Choose whether your institution uses 2 × Na or 2 × (Na + K).
• Click Calculate to get calculated osmolality and osmolar gap.

Units matter. This calculator expects conventional chemistry units (mg/dL for glucose, BUN, ethanol; mEq/L for sodium and potassium).

Interpreting results

Different hospitals use different reference ranges, but a common rough framework is:

• About -10 to +10 mOsm/kg: often considered within expected range.
• Greater than +10 mOsm/kg: elevated gap; consider unmeasured osmoles.
• Greater than +20 mOsm/kg: significantly elevated and more concerning for toxic alcohol exposure, depending on timing and context.

Important: a normal osmolar gap does not completely exclude serious poisoning. In late presentation, toxic alcohol may be metabolized, reducing the gap while acidosis worsens.

Common causes of an elevated osmolar gap

Toxic alcohols and related compounds

• Methanol
• Ethylene glycol
• Isopropanol
• Propylene glycol (including iatrogenic exposure from IV medications)

Other contributors and caveats

• High ethanol levels (if not included in formula)
• Laboratory or sampling variation
• Hyperlipidemia/hyperproteinemia effects on some analyzers
• Differences in formula selection between institutions

Worked example

Suppose measured osmolality is 330 mOsm/kg, Na is 140 mEq/L, glucose is 90 mg/dL, BUN is 14 mg/dL, and ethanol is 0 mg/dL using the 2 × Na formula:

Calculated osmolality = 2×140 + (90/18) + (14/2.8) + (0/3.7) = 280 + 5 + 5 + 0 = 290 mOsm/kg

Osmolar gap = 330 − 290 = 40 mOsm/kg, which is markedly elevated and would typically prompt urgent clinical evaluation.

Limitations you should know

• “Osmolar gap” and “osmolal gap” are often used interchangeably in practice, but measured tests are osmolality-based.
• No single formula is universally accepted.
• Timing of ingestion matters; early and late presentations can look very different.
• Always pair this with acid-base analysis, anion gap, lactate, renal function, and toxicology data.

Bottom line

This osmolar gap calculator is a fast educational tool to support interpretation of lab data. It is not a diagnostic substitute for emergency or toxicology consultation. If toxic ingestion is suspected, seek immediate medical care and follow local clinical protocols.