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 stool 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, 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[5] and villous adenoma.
References
- ^ "Diarrhea: Approach to the Patient With Lower GI Complaints: Merck Manual Professional". Retrieved 2009-04-10.
- ^ 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.
- ^ 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.
- ^ Ghosh, Amit K.; Habermann, Thomas (2007). Mayo Clinic Internal Medicine Concise Textbook. Informa Healthcare. p. 228. ISBN 1-4200-6749-4.
- ^ Oster, JR; Materson, BJ; Rogers, AI (1980). "Laxative abuse syndrome". The American Journal of Gastroenterology. 74 (5): 451–8. PMID 7234824.