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).[1] The 290 is the value of the plasma osmolality. The stool osmolality is usually not directly measured, and is often given a constant in the range of 290 to 300.[2]

A low stool osmolic gap can imply secretory diarrhea, while a high gap can imply osmotic diarrhea.[3] 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.[4]

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[5] and villous adenoma.

References

  1. ↑ "Diarrhea: Approach to the Patient With Lower GI Complaints: Merck Manual Professional". Retrieved 2009-04-10.
  2. ↑ Topazian M, Binder HJ (May 1994). "Brief report: factitious diarrhea detected by measurement of stool osmolality". N. Engl. J. Med. 330 (20): 1418–9. doi:10.1056/NEJM199405193302004. PMID 8159195.
  3. ↑ Shiau YF, Feldman GM, Resnick MA, Coff PM (June 1985). "Stool electrolyte and osmolality measurements in the evaluation of diarrheal disorders". Ann. Intern. Med. 102 (6): 773–5. doi:10.7326/0003-4819-102-6-773. PMID 3994188.
  4. ↑ Ghosh, Amit K.; Habermann, Thomas (2007). Mayo Clinic Internal Medicine Concise Textbook. Informa Healthcare. p. 228. ISBN 1-4200-6749-4.
  5. ↑ Oster, JR; Materson, BJ; Rogers, AI (1980). "Laxative abuse syndrome". The American journal of gastroenterology 74 (5): 451–8. PMID 7234824.
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