Osmolal Gap Calculator
Compare measured serum osmolality with the calculated value and see whether an unmeasured solute may be present.
What it does
Serum osmolality is a count of the dissolved particles in blood. Laboratories measure it from the freezing point of the sample, while clinicians often estimate it from sodium, glucose and urea because those three account for most of the osmolality in ordinary blood. The difference between the measured and calculated values is the osmolal gap.
The gap exists because the calculation includes only the particles that are easy to measure. Mannitol, methanol, ethylene glycol, isopropanol and other small molecules can raise the measured osmolality without moving sodium, glucose or urea. A widening gap is therefore a clue that something else is in the blood, which is why the test appears in toxicology and emergency algorithms.
The common US formula is 2 x sodium plus glucose divided by 18 plus BUN divided by 2.8. The divisors convert milligrams per deciliter into millimoles per liter, so the result is in mOsm/kg. A laboratory that reports glucose and urea in SI units needs a different form, and the result should not be mixed with a value built from mg/dL.
Reference values are not perfectly uniform. A 2020 study of healthy blood donors found a narrow range around zero and suggested an upper reference near 5 mOsm/kg, while also recommending that poisoning be suspected at 20 mOsm/kg or more. Age, kidney function, sampling delay and the laboratory method all move the number, so the bands here are practical guides rather than sharp cutoffs.
Use the gap with the history. A normal gap does not exclude every toxic alcohol, because the gap can be small early after ingestion and because some laboratories measure osmolality in a way that changes the baseline. A high gap with an unexplained acidosis, visual symptoms or a suspicious history needs urgent toxicology input.
How to use
- Enter the serum sodium in mmol/L.
- Enter glucose and BUN in mg/dL. Convert mmol/L values first if needed.
- Enter the measured osmolality from the same sample.
- Read the calculated osmolality, the gap and the band.
- Compare the gap with the clinical picture and with any previous values.
- If the gap is 20 mOsm/kg or more, treat it as a reason for urgent clinical review, not as a stand alone diagnosis.
Formula
- Sodium
- Serum sodium in mmol/L; multiplied by 2 to account for its accompanying anions.
- Glucose
- Serum glucose in mg/dL, divided by 18 to convert to mmol/L.
- BUN
- Blood urea nitrogen in mg/dL, divided by 2.8 to convert to mmol/L.
- Gap
- Measured osmolality minus the calculated value, in mOsm/kg.
The 2 x sodium term is an approximation that bundles sodium with its associated chloride and bicarbonate. The glucose and BUN divisors are unit conversions, not clinical constants: 18 mg/dL is about 1 mmol/L of glucose, and 2.8 mg/dL of urea nitrogen is about 1 mmol/L. A common SI form is 1.86 x sodium + glucose + urea, with all three in mmol/L, and the Oslo reference study divides that sum by 0.93 to report mOsm/kg. Do not compare a gap built with one form against a reference range built with the other.
Worked example
Example: sodium 140, glucose 90, BUN 14, measured osmolality 290
Defaults: sodium 140 mmol/L, glucose 90 mg/dL, BUN 14 mg/dL and measured osmolality 290 mOsm/kg.
- Double the sodium: 2 x 140 = 280.
- Convert glucose: 90 / 18 = 5.
- Convert BUN: 14 / 2.8 = 5.
- Add the parts: 280 + 5 + 5 = 290 mOsm/kg calculated.
- Subtract: 290 measured - 290 calculated = 0 mOsm/kg.
- The gap is under 10, so the tool shows 290.0 mOsm/kg, a gap of 0.0 mOsm/kg and the band Normal.
Reference table
| Osmolal gap | Band | What it suggests |
|---|---|---|
| Below 10 mOsm/kg | Normal | The measured and calculated values agree within the usual range |
| 10 to 19 mOsm/kg | Borderline | Repeat the sample and review the history, medicines and kidney function |
| 20 mOsm/kg or more | High | Consider an unmeasured solute such as mannitol, methanol or ethylene glycol |
| Negative gap | Normal | Usually inside the normal band, though a delayed sample, a laboratory method or a unit mismatch can produce a negative value |
Limits
- A normal gap does not exclude every toxic alcohol, especially early after ingestion or when the osmolality was measured late.
- The formula assumes glucose and BUN are reported in mg/dL; SI values need a different equation.
- Mannitol, contrast and some intravenous fluids can raise the gap without poisoning.
- The measured value depends on the laboratory method, sample handling and time between collection and analysis.
Common mistakes
- Mixing mg/dL glucose with the mmol/L formula, or the reverse, which shifts the gap by a large amount.
- Using a measured osmolality from a different sample than the sodium, glucose and BUN.
- Treating a gap of 10 to 19 as proof of poisoning when it is a borderline result that needs context.
- Forgetting mannitol or other unmeasured solutes that a patient received in the hospital.
- Reading a normal gap as a guarantee in a patient with a strong history and an unexplained acidosis.
Try it yourself
Quick checks. Answers sit tucked under each question.
Sodium 145, glucose 180, BUN 28 and measured osmolality 310. What is the gap?
2 x 145 = 290, 180 / 18 = 10 and 28 / 2.8 = 10. The calculated value is 310.0 mOsm/kg, so the gap is 0.0 mOsm/kg.
Sodium 135, glucose 90, BUN 14 and measured osmolality 320. What is the gap and band?
2 x 135 = 270, 90 / 18 = 5 and 14 / 2.8 = 5. The calculated value is 280.0 mOsm/kg, so the gap is 40.0 mOsm/kg, which is in the high band.
Sodium 150, glucose 300, BUN 20 and measured osmolality 330. What is the gap?
2 x 150 = 300, 300 / 18 = 16.7 and 20 / 2.8 = 7.1. The calculated value is 323.8, so the gap is 6.2 mOsm/kg, which is normal.
Questions
What is the difference between osmolality and osmolarity?
Osmolality is particles per kilogram of solvent, which is what laboratories measure. Osmolarity is particles per liter of solution. In blood the two numbers are close, and the gap is usually reported in mOsm/kg.
Why does the formula use 2 x sodium?
Sodium is the main extracellular cation, and each sodium ion is balanced by anions such as chloride and bicarbonate. Doubling sodium approximates the contribution of both sides of the salt.
What causes a high osmolal gap?
Unmeasured solutes such as mannitol, methanol, ethylene glycol, isopropanol or propylene glycol can raise it. Severe kidney failure, shock and some laboratory issues can also change the measured or calculated value.
Can the gap be negative?
Yes. A negative value can come from a delay in measuring the sample, from the laboratory method, or from a calculation that used the wrong units. It is not automatically a sign of disease.
Is a gap of 15 dangerous?
A borderline value is not a diagnosis by itself. It should be repeated and read with the history, the acid base status and the medicines the person received. A value of 20 or more is the more concerning band.
Does this replace a toxic alcohol blood test?
No. The gap is a screening clue while a specific test is pending. If poisoning is suspected, treatment should not wait for the gap or for a send out laboratory result.
Sources
- Reference values for osmolal gap in healthy subjects and medical inpatients, PubMed
- Serum osmolality and hyperosmolar states, Pediatric Nephrology, PubMed