Stool Osmotic Gap
Separates osmotic from secretory diarrhea using stool sodium and potassium.
When to use
Adults with chronic diarrhea when distinguishing osmotic from secretory causes helps the workup.
Pearls and pitfalls
- The gap uses 290 mOsm/kg as the reference fecal osmolality.
- A gap below 50 suggests osmotic diarrhea; above 100 suggests secretory.
- The test is most useful in watery diarrhea without blood or pus.
- The result does not replace assessment of infection, inflammation and structural causes.
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Formula
Gap = 290 − 2 × (stool sodium + stool potassium).
Evidence
Calculation derived from diarrhea physiology studies to separate osmotic from secretory causes.
References
- Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea. Gastroenterology. 1992;103(2):545-551. doi:10.1016/0016-5085(92)90845-4
Frequently asked questions
What is Stool Osmotic Gap?
Separates osmotic from secretory diarrhea using stool sodium and potassium. Adults with chronic diarrhea when distinguishing osmotic from secretory causes helps the workup.
How is the result calculated?
Gap = 290 − 2 × (stool sodium + stool potassium).
How should the result be interpreted?
Calculation derived from diarrhea physiology studies to separate osmotic from secretory causes.
What are the sources?
Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea. Gastroenterology. 1992;103(2):545-551.