How is plasma osmolality determined?
Plasma osmolality can be calculated with the following equation:
As the equation implies, serum sodium concentration is by far the main determinant of plasma osmolality. Plasma osmolality can be directly measured with an osmometer. An osmolar gap exists if there is greater than a 10 mOsm/kg H 2 O discrepancy between the calculated and measured osmolality, indicative of the presence of a solute not routinely measured in plasma.
