How do you calculate serum osmolality?
Double the sodium, then add glucose and urea in mmol/L. In health these three account for almost all the dissolved particles in plasma.
mmol/L: calculated osmolality = 2 × Na + glucose + urea
Urea in mg/dL: calculated osmolality = 2 × Na + glucose ÷ 18 + urea ÷ 6
BUN in mg/dL: calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8
Osmolal gap = measured osmolality − calculated osmolality
Sodium is always in mmol/L. Dividing by 18, 6 and 2.8 converts glucose, urea and BUN from mg/dL to mmol/L. The calculated osmolality is shown as a whole number. The gap is shown to up to one decimal and uses the unrounded calculated value.
How to use the calculator
- Choose the Glucose and urea units: “mmol/L (glucose, urea)”, “mg/dL (glucose, urea)” or “mg/dL (glucose, BUN)”. It’s worth checking whether your report says urea or BUN.
- Enter Sodium, Glucose and Urea or BUN from the same sample.
- If the laboratory measured osmolality, enter it in Measured osmolality (optional) to get the Osmolal gap. Otherwise leave it empty.
Worked examples
Example 1 – mmol/L, with a measured value. Na 141, glucose 6, urea 7 mmol/L; measured osmolality 300 mOsm/kg. 2 × 141 + 6 + 7 = 282 + 13 = 295 mOsm/kg. Gap: 300 − 295 = 5 mOsm/kg.
Example 2 – urea in mg/dL, raised gap. Na 140 mmol/L, glucose 100 mg/dL, urea 43 mg/dL; measured osmolality 315 mOsm/kg. 2 × 140 + 100 ÷ 18 + 43 ÷ 6 = 280 + 5.56 + 7.17 = 293 mOsm/kg (292.7 before rounding). Gap: 315 − 292.7 = 22.3 mOsm/kg, well above the usual range. Something the formula does not count is contributing.
Example 3 – BUN in mg/dL, raised gap. Na 138 mmol/L, glucose 90 mg/dL, BUN 14 mg/dL; measured osmolality 312 mOsm/kg. 2 × 138 + 90 ÷ 18 + 14 ÷ 2.8 = 276 + 5 + 5 = 286 mOsm/kg. Gap: 312 − 286 = 26 mOsm/kg.
Example 4 – hyperglycaemia. Na 130, glucose 30, urea 12 mmol/L. 2 × 130 + 30 + 12 = 260 + 42 = 302 mOsm/kg. Sodium is low, yet osmolality is high because of the glucose. The same patient in mg/dL (glucose 540, urea 72) gives the same 302. To see what the sodium will do as glucose falls, use the corrected sodium calculator.
Unit conversions
| From | To | Operation |
|---|---|---|
| Glucose mmol/L | Glucose mg/dL | × 18 |
| Glucose mg/dL | Glucose mmol/L | ÷ 18 |
| BUN mg/dL | Urea mmol/L | ÷ 2.8 |
| Urea mg/dL | Urea mmol/L | ÷ 6 |
| Urea mg/dL | BUN mg/dL | ÷ 2.14 |
A serum osmolality of roughly 275–295 mOsm/kg is a commonly quoted reference range; it’s best to check your laboratory’s values.
Common mistakes
- Swapping urea and BUN. Urea in mg/dL is about 2.14 times BUN. In Example 2, entering urea 43 as BUN gives 301 instead of 293, and the gap shrinks from 22.3 to 14.1. It’s worth checking the name and the reference range on the report.
- Mixing units. The unit choice applies to glucose and urea together. Glucose in mg/dL with urea in mmol/L gives a wrong result.
- Using samples taken at different times. Measured and calculated osmolality must come from the same sample, or the gap is meaningless.
- Confusing osmolality with tonicity. Urea crosses cell membranes freely. It adds to osmolality but not to the “effective” osmolality (tonicity) that moves water between cells and blood.
Limits of the method
- Ethanol is not in the formula. It is a common cause of a raised gap, and some references add a separate ethanol term.
- A normal gap does not exclude poisoning. Late after a toxic alcohol ingestion the parent compound is already metabolised: the osmolal gap may be normal while the anion gap is high.
- Severe hyperlipidaemia or hyperproteinaemia can affect the measured sodium and so the calculated value.
- Different formulas exist, with slightly different coefficients, so the “normal” gap depends on which one you use.
For the bigger picture in acid–base problems, see the arterial blood gas guide.
Frequently asked questions
What is the formula for calculated osmolality?
Calculated osmolality = 2 × Na + glucose + urea, with everything in mmol/L. With mg/dL values, divide glucose by 18 and urea by 6, or BUN by 2.8, to convert them first. The calculator offers all three unit options and gives the result in mOsm/kg.
Why is sodium doubled?
Because each sodium ion comes with an anion, mainly chloride or bicarbonate. Doubling sodium is a simple way to count both without measuring every anion. That is why sodium dominates the result: with Na 140 mmol/L, the 280 from 2 × Na makes up more than 90% of a normal calculated osmolality.
What is the osmolal gap?
The osmolal gap is measured osmolality minus calculated osmolality. A large gap means the blood contains osmotically active substances the formula does not count, such as ethanol, methanol, ethylene glycol or mannitol. You need a measured value from the laboratory osmometer to work it out.
What is a normal osmolal gap?
Below about 10 mOsm/kg is the figure most often quoted. The range in healthy people is wide, though, and it varies with the formula and the laboratory. A normal gap does not rule out toxic alcohol poisoning, especially late after ingestion, when the anion gap may be the clearer clue.
Is urea the same as BUN?
No, though they describe the same molecule. Urea is the whole molecule; BUN (blood urea nitrogen) counts only its nitrogen. Urea in mg/dL is about 2.14 times BUN: typical ranges are roughly 15–50 mg/dL for urea and 7–23 mg/dL for BUN. BUN in mg/dL ÷ 2.8 gives urea in mmol/L.
What is the difference between osmolality and osmolarity?
Osmolality is per kilogram of water; osmolarity is per litre of solution. The laboratory osmometer measures osmolality. The calculated value is strictly an osmolarity, but in plasma the difference is small, so it is used as an estimate of osmolality and compared with the measured value.