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Serum osmolal gap

Michael Emmett, MD
Biff F Palmer, MD
Section Editor
Richard H Sterns, MD
Deputy Editor
John P Forman, MD, MSc


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, 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 [1-5]:

 Calculated Sosm   =   (2  x  serum [Na, in mmol/L])  +  [glucose, in mg/dL]/18  + [blood urea nitrogen, 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 (calculator 1 and calculator 2) [6].

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Literature review current through: Nov 2017. | This topic last updated: Jun 20, 2017.
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