<unk>  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<unk>  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 (as reflected in the formulas for serum osmol<unk>  and anion<unk> , while the digestive tract contains significant amounts of other compounds.<unk>  is a measure of the concentration of those other compounds.<unk>  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 normal<unk>  is between 50 and 100 mOsm/kg, corresponding to the concentration of other solutes such as magnesium salts and sugars. A low<unk>  suggests secretory diarrhea, wherein the digestive tract is hyperpermeable and losing electrolytes, while a high<unk>  suggests<unk>  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<unk>  diarrhea, other molecules such as unabsorbed carbohydrates are more significant contributors to stool osmolality. High<unk> <unk>  (>100 mOsm/kg) causes of<unk>  diarrhea include celiac sprue, chronic pancreatitis, lactase deficiency, lactulose,<unk>  laxative use/abuse, and Whipple's disease. Low<unk> <unk>  (<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.
Stool osmotic gap
Bristol stool scale
Fecal occult blood
D-xylose absorption test
Urine anion gap
Sucrose gap
Fecal pH test
Stool test