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Stool osmotic gap is a measurement of the difference in solute types between serum and feces, used to distinguish among different causes of diarrhea.
Feces is normally in osmotic equilibrium with blood serum, which the human body maintains between 290–300 mOsm/kg. However, the solutes contributing to this total differ. Serum is mostly sodium and potassium salts , while the digestive tract contains significant amounts of other compounds. Stool osmotic gap is a measure of the concentration of those other compounds.
Stool osmotic gap is calculated as 290 mOsm/kg − 2 ×. 290 mOsm/kg is the presumed stool osmolality, and the measured concentration of sodium and potassium cations is doubled to account for the corresponding anions which must be present.
A normal gap is between 50 and 100 mOsm/kg, corresponding to the concentration of other solutes such as magnesium salts and sugars.