The osmolar gap is used to identify unmeasured compounds in serum or plasma when present in high molar concentrations. If the plasma ethanol concentration is available, please use “serum osmolar gap (including ethanol) calculator”. The equation is based on the difference between measured osmolality and calculated osmolality.


Serum osmolar gap = measured osmolality – 2 x Sodium – urea – glucose (all results in mmol/L except measured osmolality in mmol/kg)

Osmolality range is 200-450 mmol/kg
Sodium range is 80-190 mmol/L
Urea range is 0.0-100.0 mmol/L
Glucose range is 1-150 mmol/L
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Serum Osmolar Gap


Serum Osmolar Gap reference interval is 0 to +10 mmol/L Interpretation A raised osmolar gap may be caused by high concentrations of ethanol, methanol, ethylene glycol, mannitol or glycine. The osmolar gap may also be high in pseudohyponatraemia caused by very higher serum lipids or serum protein.