Twice the sodium plus urea and glucose
Expansion
Calculated osmolality = 2 x sodium + urea + glucose, all in mmol/l. Doubling sodium accounts for its accompanying anions.
Normal is 275 to 295 mosmol/kg.
Osmolality versus osmolarity: osmolality is per kilogram of solvent and is what the laboratory measures; osmolarity is per litre of solution. The difference is negligible in practice.
Tonicity is not the same as osmolality. Only effective osmoles, which cannot cross cell membranes, cause water to shift.
- Effective: sodium, glucose (in the absence of insulin), mannitol
- Ineffective: urea and ethanol, which equilibrate freely
That is why uraemia raises measured osmolality without causing cellular dehydration, and why urea is excluded when assessing true tonicity.
The osmolar gap = measured - calculated, normally under 10.
A raised gap means unmeasured osmoles are present: methanol, ethylene glycol, ethanol, mannitol, propylene glycol, or pseudohyponatraemia from severe hyperlipidaemia or paraproteinaemia.
In toxic alcohol poisoning the gaps move in sequence: early, a high osmolar gap with a normal anion gap, as the parent alcohol circulates; later, a normal osmolar gap with a high anion gap, once it has been metabolised to acid. A normal osmolar gap late therefore does not exclude poisoning.