OSMOLAL GAP

Osmolal gap = Measured osmolality (by freezing point depression) - Calculated osmolality

• Calculated osmolality = easily measurable osmotic substances
• Calculated osmolality = Na + Cl + Urea + Glucose (in mmol/L) (~ 2 x Na + Urea + Glucose)
• Osmolal gap = non directly measurable osmotic substances

Causes for widening of osmolal gap i.e. measured osmolality greater than calculated osmolality

• Endogenous sources - lactate, ketoacid
• Exogenous sources - mannitol, ethanol, ethylene glycol, methanol