Osmolal gap = measured serum osmolality – calculated osmolality
Calculated osmolality = (2 x [Na]) + [Glucose] + [Urea] (all in mmol/L)
A normal osmolal gap is < 10mOsm/kg
When the osmolal gap is increased (>10 mOsm/kg), causes to consider include:
- Alcohols e.g. ethanol, methanol, ethylene glycol
- Sugars e.g. mannitol
- Lipids e.g. hypertriglyceridaemia
- Proteins e.g. Waldenstrom macroglobulinaemia
