Short answer
Stool electrolytes measure sodium and potassium in watery stool. They can be used to calculate a stool osmotic gap, a clue that helps separate osmotic diarrhea from secretory diarrhea in selected chronic watery diarrhea workups. This is a targeted GI test, not a microbiome wellness score.
How the pattern is used
| Pattern | Possible meaning | Examples of follow-up context |
|---|---|---|
| High osmotic gap | Can fit osmotic diarrhea from poorly absorbed solutes. | Carbohydrate malabsorption, magnesium products, sugar alcohols, laxatives, or malabsorption questions. |
| Low osmotic gap | Can fit secretory diarrhea with electrolyte-rich stool water. | Bile acid diarrhea, some infections, endocrine causes, inflammatory or medication-related patterns. |
| Mixed or borderline | Real patients may not fit a clean textbook category. | Diet, fasting response, medications, stool handling, and other tests matter. |
| Formed stool | Usually not the right specimen for this question. | Testing is most relevant to watery diarrhea. |
Limits
The osmotic gap is a clue, not a diagnosis. Collection quality, contamination, stool storage, laxative use, and diet can change interpretation. Many chronic diarrhea workups also consider infection testing, celiac testing, bile acid diarrhea, fecal calprotectin, pancreatic elastase, thyroid disease, medication effects, and colonoscopy when indicated.
What follow-up may matter
In practice, the osmotic gap is most helpful when the stool story is already watery and chronic. A high gap can push the conversation toward carbohydrate malabsorption or laxative effects, while a low gap can push it toward secretory causes or bile acid diarrhea. Either way, the next step is usually not "optimize the gut"; it is to find the actual cause of the diarrhea.
Questions to ask
- Is the diarrhea truly chronic and watery, or is another stool test more appropriate?
- Were stool sodium and potassium measured, and how was the osmotic gap calculated?
- Did symptoms improve with fasting or dietary change?
- Should laxatives, magnesium, sugar alcohols, bile acid diarrhea, infection, celiac disease, or IBD be evaluated?
FAQ
What does a high stool osmotic gap suggest?
It can fit osmotic diarrhea from unabsorbed solutes such as carbohydrate malabsorption or certain laxatives.
What does a low stool osmotic gap suggest?
It can fit secretory diarrhea, such as bile acid diarrhea or some infectious, endocrine, or medication-related patterns.
Can this test diagnose the cause by itself?
No. It is a clue that needs to be combined with the rest of the diarrhea workup.
Does fasting help interpretation?
Sometimes symptom response to fasting helps the clinician think through osmotic versus secretory patterns.
Why does stool handling matter so much?
Contamination, storage, and sample quality can change the measured electrolytes and the calculated gap.
What tests often come next?
Depending on the story, clinicians may consider celiac testing, infection tests, bile acid diarrhea evaluation, calprotectin, or pancreatic elastase.
Related guides: bile acid malabsorption testing, fructose breath test, lactose intolerance breath test, and fecal calprotectin.