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
- Biff F Palmer, MD
Biff F Palmer, MD
- Professor of Internal Medicine
- University of Texas Southwestern Medical Center
DETERMINANTS OF SERUM OSMOLALITY
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) .
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- DETERMINANTS OF SERUM OSMOLALITY
- MEASUREMENT OF SERUM OSMOLALITY
- CLINICAL UTILITY OF THE SERUM OSMOLAL GAP
- MAJOR CAUSES OF AN ELEVATED 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