Calculate the serum osmolar gap using measured serum osmolality, sodium, glucose, and BUN. Learn the formula, example calculation, normal range, clinical significance, and interpretation of an elevated osmolar gap.
What Is an Osmolar Gap?
The osmolar gap, also called the osmolal gap, is the difference between the measured serum osmolality and the calculated serum osmolality. It is used to identify a difference between the osmotically active substances actually measured in serum and those estimated from the routine laboratory values used in the calculation.
An increased osmolar gap may occur when unmeasured osmotically active substances are present in the blood. In the appropriate clinical setting, this can provide an important clue to the presence of certain toxic alcohols and other abnormal solutes.
Osmolar Gap Formula
When sodium, glucose, and BUN are reported in conventional units, calculated serum osmolality can be estimated using:
Where sodium is expressed in mmol/L (numerically equivalent to mEq/L), glucose and BUN are expressed in mg/dL, and calculated osmolality is reported in mOsm/kg.
Example Calculation
| Parameter | Result |
|---|---|
| Serum Sodium | 140 mmol/L |
| Glucose | 90 mg/dL |
| BUN | 14 mg/dL |
| Measured Serum Osmolality | 305 mOsm/kg |
Step 1: Calculate serum osmolality
Calculated Osmolality = (2 × 140) + (90/18) + (14/2.8)
Calculated Osmolality = 280 + 5 + 5
Calculated Osmolality = 290 mOsm/kg
Step 2: Calculate the osmolar gap
Osmolar Gap = 305 − 290
Osmolar Gap = 15 mOsm/kg
Osmolar Gap Calculator
Calculate the osmolar gap quickly using sodium, glucose, BUN, and measured serum osmolality.
Open Osmolar Gap Calculator →Normal Osmolar Gap
The expected osmolar gap depends on the laboratory method, analytical variation, and the formula used to calculate serum osmolality.
A commonly used clinical reference is approximately:
Some clinical references and laboratories may use a wider upper limit, such as approximately 10–15 mOsm/kg. The laboratory-specific reference interval should therefore be considered when interpreting the result.
What Does an Increased Osmolar Gap Mean?
An elevated osmolar gap indicates that measured serum osmolality is higher than the calculated value by more than expected from analytical and biological variation.
Possible causes include the presence of unmeasured osmotically active substances. Important clinical causes include certain toxic alcohol exposures.
- Methanol
- Ethylene glycol
- Isopropanol
- Ethanol and other alcohols
- Other unmeasured osmoles
An elevated osmolar gap is not specific for toxic alcohol poisoning. The result must be interpreted together with the patient's history, symptoms, acid-base status, anion gap, renal function, and other laboratory findings.
Osmolar Gap and Toxic Alcohols
The osmolar gap can be useful as part of the laboratory assessment of suspected toxic alcohol exposure. Early after ingestion of some toxic alcohols, the parent compound may contribute substantially to the osmolar gap. As metabolism occurs, acidic metabolites may accumulate and the anion gap can become more prominent.
Therefore, a normal osmolar gap does not completely exclude toxic alcohol poisoning, particularly when presentation occurs later after ingestion.
When toxic alcohol exposure is suspected, direct measurement of the relevant alcohols and urgent clinical evaluation may be required.
Osmolar Gap vs Anion Gap
| Osmolar Gap | Anion Gap |
|---|---|
| Compares measured and calculated serum osmolality | Estimates the concentration of commonly unmeasured serum anions |
| Reported in mOsm/kg | Reported in mEq/L |
| May provide a clue to unmeasured osmoles | May provide a clue to unmeasured anions |
| Useful in selected toxicology and metabolic evaluations | Commonly used in acid-base assessment |
Why Is BUN Included in the Calculation?
BUN contributes to measured serum osmolality and is therefore included in the commonly used calculated osmolality equation. When BUN is reported in mg/dL, it is divided by 2.8 to convert its contribution into the corresponding mmol/L-based term used in the calculation.
Important Unit Requirements
Correct units are essential for accurate calculation.
| Parameter | Required Unit |
|---|---|
| Sodium | mmol/L |
| Glucose | mg/dL |
| BUN | mg/dL |
| Measured Osmolality | mOsm/kg |
If glucose or BUN is reported in SI units such as mmol/L, an appropriate formula using those units must be used rather than directly applying the mg/dL equation.
Limitations of the Osmolar Gap
- The osmolar gap is not specific for toxic alcohol exposure.
- Different formulas for calculated osmolality can produce slightly different results.
- Laboratory analytical variation can affect the calculated gap.
- A normal osmolar gap does not completely exclude toxic alcohol poisoning.
- The gap should be interpreted with the clinical history and other laboratory findings.
When there is significant clinical suspicion of poisoning, treatment decisions should not be based on the osmolar gap alone.
How to Calculate Osmolar Gap
- Obtain the serum sodium concentration.
- Obtain serum glucose and BUN values.
- Calculate serum osmolality using the appropriate formula.
- Obtain measured serum osmolality from the laboratory result.
- Subtract calculated osmolality from measured osmolality.
- Interpret the result using the laboratory reference interval and clinical context.
Frequently Asked Questions
What is an osmolar gap?
The osmolar gap is the difference between measured serum osmolality and calculated serum osmolality.
What is the formula for osmolar gap?
Osmolar Gap = Measured Serum Osmolality − Calculated Serum Osmolality.
How is calculated serum osmolality determined?
A commonly used formula is 2 × sodium + glucose/18 + BUN/2.8 when sodium is in mmol/L and glucose and BUN are in mg/dL.
What is a normal osmolar gap?
A commonly used reference is less than about 10 mOsm/kg, although laboratory-specific limits may differ and some references use approximately 10–15 mOsm/kg as the upper range.
Does an increased osmolar gap mean toxic alcohol poisoning?
No. An increased osmolar gap can occur for several reasons. Toxic alcohol exposure is an important consideration in the appropriate clinical setting, but the result is not diagnostic by itself.
Can toxic alcohol poisoning occur with a normal osmolar gap?
Yes. The osmolar gap can change over time as toxic alcohols are metabolized. Therefore, a normal osmolar gap does not completely exclude toxic alcohol poisoning.
Related Calculator
Osmolar Gap Calculator
Use the LabProSuite calculator to calculate serum osmolar gap from routine laboratory values and measured osmolality.
Calculate Osmolar Gap →Medical Disclaimer: This calculator and article are intended for educational and clinical support purposes only. Results should be interpreted by qualified healthcare professionals together with the patient's clinical condition and other laboratory findings. They should not replace professional medical advice, diagnosis, or treatment.
