The normal plasma osmolality is 285-295 mOsm/kg. This is value is determined by multiple plasma solutes including sodium, urea and glucose. The equation below is used to estimate plasma osmolality.
Osmolality = 2×[Na] + [glucose]/18 + [BUN]/2.8
Though not explicitly stated in the formula above, the doubling of the sodium concentration is used to indicate the effect of associated anions like bicarbonate and chloride.
It is worth mentioning here that the tonicity or effective osmolality of the plasma is governed primarily by sodium and glucose. This value essentially determines the osmotic force impacting the shift of water between the intracellular compartment and the extracellular compartment. This equation is: Tonicity=2×[Na]+[glucose]/18
Updated definition 2020:
The serum (or plasma) osmolality is determined by the concentrations (in mmol/L) of the different solutes in the plasma. In normal subjects, sodium salts (mainly chloride and bicarbonate), glucose, and urea are the primary circulating solutes. If no other solutes are present in serum at high millimolar concentrations (greater than 5 mmol/L), then these three solute concentrations can be used to predict the measured osmolality. A variety of formulas have been evaluated for this purpose, and the following relationship is acceptable for practical clinical purposes:
Calculated Sosm= (2 x serum Na [in mmol/L]) + [Glucose, in mg/dL]/18 + [BUN, in mg/dL]/2.8
Or, with international units (all of which are in mmol/L):
Calculated Sosm= (2 x serum [Na]) + [glucose] + [urea]
The serum sodium is multiplied by two to account for the osmolal contributions of the accompanying anions (chloride and bicarbonate), and, in the first formula, the divisors 18 and 2.8 convert units of mg/dL to mOsmol/kg
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