What is the calculated osmolality formula?

The common US formula is 2 × Na + glucose/18 + BUN/2.8, with sodium in mmol/L and glucose and BUN in mg/dL. The SI version is 2 × Na + glucose + urea with everything in mmol/L. The 2 × Na term accounts for the accompanying anions.

What is the osmol gap?

The osmol gap is the measured osmolality minus the calculated osmolality. A normal gap is roughly −10 to +10 mOsm/kg. A raised gap means unmeasured osmotically active particles are present, such as toxic alcohols, mannitol, or ketones.

Does a normal osmol gap rule out toxic alcohol poisoning?

No. The osmol gap can be normal late after ingestion once the parent alcohol has been metabolised to its acid metabolite, even though the patient is now most toxic. Always combine it with the anion gap, history, and timing.

Ethanol is osmotically active and raises measured osmolality. Including the ethanol level (÷3.7 for mg/dL) in the calculated value lets you uncover an additional gap from a second toxic alcohol that ethanol alone would otherwise mask.

What causes a raised osmol gap?

Methanol, ethylene glycol, isopropanol, ethanol, propylene glycol, mannitol, glycine, and severe ketoacidosis or lactic acidosis can all raise the osmol gap. The pattern of osmol gap plus anion gap helps narrow the differential.

Serum Osmolality & Osmol Gap Calculator

The serum osmolality is the concentration of all osmotically active particles dissolved in blood. Comparing a calculated estimate against a measured value yields the osmol gap — a fast screening tool that emergency physicians, toxicologists, and nephrologists use to detect dangerous unmeasured substances such as methanol and ethylene glycol.

How it works

The calculator estimates osmolality from the three solutes that dominate it. Using US conventional units:

Calculated osmolality = 2 × Na + glucose/18 + BUN/2.8

Sodium is in mmol/L; glucose and BUN are in mg/dL. The factor 2 accounts for the anions (mostly chloride and bicarbonate) that accompany sodium. Dividing glucose by 18 and BUN by 2.8 converts mg/dL to mmol/L for those solutes. In SI units the formula simplifies to 2 × Na + glucose + urea with everything in mmol/L. If an ethanol level is entered it is added (÷3.7 for mg/dL) so a second unmeasured osmole can still be detected.

The osmol gap

Osmol gap = measured osmolality − calculated osmolality

A normal gap is roughly -10 to +10 mOsm/kg. A gap above about 10 flags unmeasured osmoles — classically the toxic alcohols (methanol, ethylene glycol, isopropanol), but also mannitol, propylene glycol, and severe ketoacidosis.

Notes and pitfalls

A normal osmol gap does not exclude toxic-alcohol poisoning. Early after ingestion the parent alcohol produces a large gap with a normal anion gap; later the alcohol is metabolised to organic acids, so the gap falls while the anion gap and acidosis worsen. The combination of both gaps over time is far more informative than either alone. All calculation runs locally in your browser.