Hong Life CalculatorsUnderstanding the serum osmolality calculator — concept and principle
Serum osmolality measures the concentration of dissolved particles in blood. The calculated value estimates it from sodium, glucose and urea, and comparing it with a laboratory-measured value gives the osmolal gap. Clinicians use these numbers to assess sodium and water disorders, to evaluate hyperglycaemic emergencies, and to screen for unmeasured substances such as toxic alcohols.
How it is calculated
Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8
Osmolal gap = measured osmolality − calculated osmolality
- Sodium in mmol/L; it is doubled to account for its accompanying anions (chloride and bicarbonate).
- Glucose and blood urea nitrogen (BUN) in mg/dL. Dividing by 18 and 2.8 converts them to mmol/L (glucose 180 g/mol; BUN counted as two nitrogen atoms, 28 g/mol).
- In SI units: 2 × Na + glucose + urea, all in mmol/L.
- Enter a measured osmolality to see the gap; enter 0 to skip it.
The principle
Osmolality depends on the number of particles, not their size. In plasma almost all particles are sodium salts, glucose and urea, so these three predict the measured value well. If something else that is osmotically active is present in large amounts, such as ethanol, methanol, ethylene glycol or mannitol, the laboratory measurement by freezing-point depression will be higher than the calculation, and the difference shows up as a raised gap.
Worked example
- 2 × Na = 2 × 140 = 280.
- Glucose ÷ 18 = 100 ÷ 18 = 5.56.
- BUN ÷ 2.8 = 14 ÷ 2.8 = 5.00.
- Calculated osmolality = 280 + 5.56 + 5.00 = 290.56 mOsm/kg.
- Measured osmolality was left at 0, so the osmolal gap is shown as —.
| Sodium (mmol/L) | Calculated osmolality (mOsm/kg) |
|---|---|
| 35 | 80.56 |
| 70 | 150.56 |
| 105 | 220.56 |
| 140 | 290.56 |
| 175 | 360.56 |
| 210 | 430.56 |
| 280 | 570.56 |
How to read the result
Normal serum osmolality is about 275–295 mOsm/kg. Higher values occur with dehydration, hypernatraemia and severe hyperglycaemia; lower values usually reflect hyponatraemia with excess water. An osmolal gap under 10 is generally considered normal. A larger gap suggests an unmeasured osmole, most commonly ethanol, and in the right context it raises concern about methanol or ethylene glycol poisoning.
Common mistakes
- Using urea in mg/dL instead of BUN; urea in mg/dL must be divided by about 6, not 2.8.
- Comparing a calculated value with a measured value taken at a different time.
- Assuming a normal gap rules out toxic alcohols; late in poisoning the gap can be normal.
Frequently asked questions
What is the difference between osmolality and osmolarity?
Osmolality is per kg of water and is what laboratories measure; osmolarity is per litre of solution.
What is effective osmolality?
It leaves out urea, which crosses cell membranes freely, so it reflects the force moving water between cells and plasma.
How much does ethanol add?
Roughly ethanol in mg/dL divided by 3.7 to 4.6, depending on the reference used.
Sources: Purssell RA et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001.