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). The 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. 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, 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 (>100 mosm/kg) causes of osmotic diarrhea include celiac sprue, chronic pancreatitis, lactase deficiency, lactulose, 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 laxative abuse and villous adenoma.