What osmolality measures
Osmolality counts the dissolved particles in a kilogram of plasma. It controls how water moves between blood and cells. Sodium and its partner anions account for most of it, followed by glucose and urea.
The formulas
In US units, with BUN and glucose in mg/dL:
- Calculated osmolality = 2 × Na + BUN ÷ 2.8 + glucose ÷ 18
- Effective osmolality (tonicity) = 2 × Na + glucose ÷ 18
- Osmolal gap = measured osmolality − calculated osmolality
The divisors 2.8 and 18 convert mg/dL to mmol/L. Effective osmolality leaves out urea because urea crosses cell membranes freely and does not move water.
Worked example
Sodium 140 mmol/L, BUN 14 mg/dL, glucose 90 mg/dL, measured osmolality 295 mOsm/kg:
- 2 × 140 = 280.
- 14 ÷ 2.8 = 5.
- 90 ÷ 18 = 5.
- Calculated osmolality: 280 + 5 + 5 = 290 mOsm/kg.
- Effective osmolality: 280 + 5 = 285 mOsm/kg.
- Osmolal gap: 295 − 290 = 5 mOsm/kg, normal.
Normal values
| Value | Usual range |
|---|---|
| Serum osmolality | 275 to 295 mOsm/kg |
| Osmolal gap | −10 to +10 mOsm/kg |
When the gap is raised
A gap above about 10 means something in the blood is not in the formula. The most common cause is ethanol: each 100 mg/dL of alcohol adds roughly 22 mOsm/kg. Other causes include methanol, ethylene glycol (antifreeze), isopropyl alcohol, mannitol and some contrast agents. A raised gap together with a raised anion gap is a classic sign of toxic alcohol poisoning, which needs urgent treatment.
High and low osmolality
High values occur with dehydration, hypernatremia, uncontrolled diabetes and kidney failure. Low values almost always reflect hyponatremia. When glucose is very high, the corrected sodium and the effective osmolality help judge the true water balance.
Limits
The constants differ slightly between sources, some formulas use 1.86 × Na instead of 2 × Na, and lab methods vary. Compare results using the same formula over time.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.