Plasma osmolarity is the number of molecules present in one liter of plasma. It is differentiated from plasma osmolality, which represents the number of molecules dissolved in one kilogram of plasma. Both values are approximately the same.
The main molecules that enter into the calculation of osmolarity are: sodium and potassium salts, glucose, and urea. In practice, there are two plasma osmolarities:
Calculated plasma osmolarity: OsmPc = (natremia x 2) + blood sugar + urea. It is normally between 280 and 295 mOsm/L.
The measured plasma osmolarity: OsmPm which corresponds to all the osmotic substances present in the plasma, including those which are not determined by a blood test. The measured plasma osmolarity is always higher than the calculated plasma osmolarity but this difference must be less than 10 mOsm/L.
Plasma osmolarity reflects the state of hydration and the amount of sodium:
Also, its measurement helps in the diagnosis of hyperosmolar coma, renal pathologies, and diabetes insipidus.
But also to follow a treatment with diuretics, to confirm the values of electrolytes, glucose, and urea, or to follow dehydration or hyperhydration.