Serum osmolal gap
- Michael Emmett, MD
Michael Emmett, MD
- Editor-in-Chief — Nephrology
- Section Editor — Fluid and Electrolytes
- Chief of Internal Medicine
- Baylor University Medical Center
DETERMINANTS AND MEASUREMENT OF SERUM OSMOLALITY
The serum (or plasma) osmolality is determined by the concentrations of the different solutes in the plasma. In normal subjects, sodium salts (chloride and bicarbonate), glucose, and urea are the primary circulating solutes. Although a variety of formulas have been evaluated to predict the plasma osmolality , most studies have concluded that serum osmolality (Sosm) can be best estimated from the following formula [1-5]:
Calculated Sosm = (2 x serum [Na]) + [glucose, in mg/dL]/18 + [blood urea nitrogen, in mg/dL]/2.8
or, with standard 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 accompanying anions (chloride and bicarbonate) and, in the first formula, the divisors 18 and 2.8 convert units of mg/dL into mosmol/kg (calculator 1) or, for standard units, (calculator 2) .
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- DETERMINANTS AND MEASUREMENT OF SERUM OSMOLALITY
- Serum osmolal gap
- ETHANOL INTOXICATION
- HIGH ANION GAP METABOLIC ACIDOSIS
- Toxic alcohols and glycols
- - Methanol and ethylene glycol
- - Other alcohols
- Ketoacidosis and lactic acidosis
- Advanced chronic kidney disease
- SUMMARY AND RECOMMENDATIONS