Serum osmolarity
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Osmolarity
Plasma osmolarity is calculated by:
2[Na+] + 2[K+] + [Glucose] + [Urea]
where all values are in mmol/l
Osmolality
- Osmolality is not typically calculated but is measured on an analytical instrument called an osmometer
- A normal osmolality is 270-310 milli-osmoles per kg
Osmolality vs osmolarity
Osmolality and osmolarity are measures that are technically different, but functionally the same for normal use. Whereas osmolality (with an l) is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg), osmolarity (with an r) is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L). Osmolarity may also use mmol/L as units
The osmolar gap is the difference between the measured osmolality and the calculated osmolarity