Calculated serum osmolality is determined from the major contributors to plasma osmolality: sodium, glucose, and urea. Comparing calculated to measured osmolality reveals the osmolal gap, useful in detecting unmeasured osmoles like ethanol, methanol, ethylene glycol, and isopropanol.
Formula
Osmolality (mOsm/kg) = 2 × Na + Glucose/18 + BUN/2.8
Where Na is in mEq/L, Glucose and BUN are in mg/dL.
Normal Range
275–295 mOsm/kg
Osmolal Gap
Osmolal Gap = Measured Osmolality − Calculated Osmolality
A gap >10 mOsm/kg suggests unmeasured osmoles. Common causes:
- Toxic alcohols (methanol, ethylene glycol, isopropanol)
- Ethanol intoxication
- Mannitol or sorbitol administration
- Severe hyperlipidemia or hyperproteinemia (pseudohyponatremia)