A calculator to identify significant numbers of unmeasured osmoles in a serum or plasma sample if ethanol result is available.
Osmolar gap (mmol/L) = measured osmolality (mmol/kg) – 2 x Sodium (mmol/L) – urea (mmol/L) – glucose (mmol/L) – 277 x ethanol (mg/dL)
Calculated Osmolality = 2 x sodium + urea + glucose + 270 x ethanol (in %*)
Calculated Osmolality = 2 x sodium + urea + glucose + 1.25 x ethanol (in mmol/L)
Note: variability in result due to uncertainty in measurements approx. +/- 7 mmol/L
* units used at SydPath and to express the legal driving limit, eg 0.05% (=11 mmol/L)
Reference interval: 0 - 10 mmol/L Interpretation A raised osmolar gap, after allowing for the presence of measured ethanol, may be caused by high concentrations of methanol, ethylene glycol, mannitol or glycine. The osmolar gap may also be high in pseudohyponatraemia caused by very higher serum lipids or serum protein.