Stool Osmolar/Osmotic Gap
Use with persistent (>4 weeks) watery diarrhea (>3 episodes/day) to guide diagnosis and treatment.
- Useful to determine whether chronic watery diarrhea is from osmotic or secretory causes.
- Not indicated in evaluation of acute diarrhea.
- Most useful in the outpatient setting, very rarely in hospital or ED.
- Initially validated in adults but used in children as well. (Eherer AJ 1992, Castro-Rodriguez JA 1997)
- A thorough history is the most helpful tool in determining the cause of chronic diarrhea.
- Osmotic diarrhea is caused when water remains in the gut lumen due to a poorly absorbed, non-electrolyte substance (Gap >100, specificity increases with higher cutoff values of 125, 160 etc). (Binder HJ 2006)
- Secretory diarrhea is due to disorders in intestinal electrolyte transport (either true secretion or poor absorption). Gap is usually <50.
- The calculator relies on the assumption that stool osmolarity is fairly constant and similar to serum osmolarity (290-300 mmol/L).
- Interpret with caution: variability in measured electrolytes has been reported in individual stool samples, and changes if the sample is left out at room temperature. (Duncan A 1992)
Distinguishing between major classes of diarrhea can be tricky. Osmotic diarrhea usually ceases with fasting and during the night whereas secretory diarrhea often persists at night and continues despite fasting. The stool gap is most helpful when reported symptoms are atypical and can guide the clinician toward a category, decreasing the cost of the diagnostic workup. The test is cheap and straightforward.
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- Osmotic diarrhea is generally treated by avoidance of the offending agent.
- Treatment for secretory diarrhea is targeted at and specific to the underlying cause.
Symptom management is also dependent on the underlying cause. Some options include:
- Increasing dietary fiber
- Low FODMAP (Fermentable Oligo-Di-Monosaccharides and Polyols) diet
- Active culture yogurt and probiotics
- Bile acid resins (cholestyramine)
- Empiric antibiotics (metronidazole, rifaximin)
- Antimotility agents (loperamide, diphenoxylate/atropine, codeine); must rule out infection first!
Stool Osmolal Gap = Stool Osm - (2 * (Na + K))
Stool Osmolal Gap = 290[mOsm/kg] - (2 * (Na + K))
Facts & Figures
- Causes of osmotic diarrhea include:
- Carbohydrate malabsorption
- Dietary (high FODMAP)
- Enteric feeding
- Osmotic laxatives
- Mg+ based
- Sorbitol ingestion
- Short gut syndrome
- Carbohydrate malabsorption
- Causes of Secretory Diarrhea include:
- Addison’s disease
- Bile acid malabsorption
- Congenital electrolyte transport disorders
- Stimulant laxatives (bisocodyl, senna)
- Small intestinal bacterial overgrowth
- Tumors (rare)
- Gastrinoma (Zollinger Ellison syndrome)
- Whipple’s disease
- Fatty diarrhea / Malabsorption / steatorrhea usually has an osmolar gap >50, but not invariably.
- Inflammatory and infectious diarrhea often have a low gap (<50) but should generally be considered as distinct clinical entities.
- Inflammatory/Infectious diarrhea are characterized by positive fecal leukocytes or other markers of inflammation in the gut like fecal calprotectin or stool lactoferrin. Very few causes in the secretory list should cause (+) fecal leukocytes.
- As an aside, infectious diarrhea is more often acute but exceptions include giardia, c.dif, e.histolytica, and parasites. These should all give (+) fecal leukocytes
- Overlap between disorders with both osmotic and secretory components is common (ex. infection, celiac disease, motility disorders, and functional disorders like irritable bowel syndrome)
Original/Primary ReferenceEherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103(2):545.
Other ReferencesSteffer KJ, Santa Ana CA, Cole JA, Fordtran JS. The practical value of comprehensive stool analysis in detecting the cause of idiopathic chronic diarrhea. Gastroenterol Clin North Am 2012;41:539-560Castro-Rodriguez JA, Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements.Arch Dis Child. 1997 Sep;77(3):201-5.Binder HJ, Causes of chronic diarrhea. N Engl J Med. 2006 Jul 20;355(3):236-9.Duncan A, Robertson C, Russell RI. The fecal osmotic gap: technical aspects regarding its calculation. J Lab Clin Med. 1992 Apr;119(4):359-63.Schiller L, Sellin J. Diarrhea. In: Sleisenger MH, Feldman M, Friedman LS, Brandt LJ, eds. Sleisenger and Fordtran’s Gastrointestinal and Liver Disease: Pathophysiology, Diagnosis, Management. 9th ed. Philadelphia, Pa.: Saunders/Elsevier; 2010:211-232.
About the Creator
John S. Fordtran, MD, is the director of gastrointestinal physiology at Baylor University Medical Center (BUMC), Texas. Previously, he was chief of internal medicine at BUMC and president of the Baylor Research Institute. Dr. Fordtran is an accomplished author of over 180 peer-reviewed articles on gastrointestinal pathophysiology, diagnosis and management and won the Janssen Award in Gastroenterology for Lifetime Achievement in Digestive Sciences.
To view Dr. John S. Fordtran's publications, visit PubMed