Stool osmotic gap

Stool osmotic gap is a calculation performed to distinguish among different causes of diarrhea.

It is calculated with the equation 290 − 2 * (stool Na + stool K).

290 is the value of the stool osmolality.

The stool osmolality is usually not directly measured, and is often given a constant in the range of 290 to 300.

A low stool osmolic gap can imply secretory diarrhea, while a high gap can imply osmotic diarrhea.

In secretory diarrhe, secreted sodium and potassium ions make up a greater percentage of the stool osmolality in secretory diarrhea, whereas in osmotic diarrhea, molecules such as unabsorbed carbohydrates are more significant contributors to stool osmolality.

A normal gap is between 50 and 100 mosm/kg.

High osmotic gap of>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 of <50 mosm/kg) causes of secretory diarrhea include toxin-mediated causes (cholera, enterotoxigenic strains of E. coli) and secretagogues such as vasoactive intestinal peptide, gastrinoma, medullary thyroid carcinoma, factitious diarrhea from non-osmotic laxative abuse and villous adenoma.

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