Stool osmotic gap
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 × (stool Na + stool K). 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 low stool osmotic gap suggests secretory diarrhea, wherein the digestive tract is hyperpermeable and losing electrolytes, while a high gap suggests osmotic diarrhea, wherein the digestive tract is unable to absorb solutes from the chyme, either because the digestive tract is hypopermeable (e.g. due to inflammation), or non-absorbable compounds (e.g. Epsom salt) are present. The reason for this is that secreted sodium and potassium ions make up a greater percentage of the stool osmolality in secretory diarrhea, whereas in osmotic diarrhea, other molecules such as unabsorbed carbohydrates are more significant contributors to stool osmolality.
High osmotic gap (>100 mOsm/kg) causes of osmotic diarrhea include celiac sprue, chronic pancreatitis, lactase deficiency, lactulose, osmotic laxative use/abuse, and Whipple's disease.
Low osmotic gap (<50 mOsm/kg) causes of secretory diarrhea include toxin-mediated causes (cholera, enterotoxigenic strains of E. coli) and secretagogues such as vasoactive intestinal peptide (from a VIPoma, for example). Uncommon causes include gastrinoma, medullary thyroid carcinoma (which produces excess calcitonin), factitious diarrhea from non-osmotic laxative abuse and villous adenoma.
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