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.