This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Sorting out chronic diarrhea
Diarrhea lasting more than four weeks has many possible causes. One useful first step is to decide whether water is being pulled into the bowel by something that should not be there (osmotic diarrhea) or actively pumped into it by the bowel wall (secretory diarrhea). The stool osmotic gap helps make this distinction from a simple stool water test.
The formula
Stool osmotic gap = 290 − 2 × (stool sodium + stool potassium)
Stool water normally has about the same osmolality as blood, around 290 mOsm/kg. Sodium and potassium, each paired with an anion, account for most of it in secretory diarrhea. Anything left over must be other dissolved particles, such as unabsorbed sugars or magnesium.
Interpretation
| Gap (mOsm/kg) | Suggests | Typical causes |
|---|---|---|
| Above 125 | Osmotic diarrhea | Lactose intolerance, sorbitol or other sugar alcohols, magnesium laxatives or antacids, polyethylene glycol, malabsorption |
| Below 50 | Secretory diarrhea | Infections and toxins, bile acid diarrhea, microscopic colitis, stimulant laxatives, hormone-secreting tumors |
| 50 to 125 | Mixed or uncertain | Often a combination of mechanisms |
A practical clue: osmotic diarrhea usually stops when the patient fasts, while secretory diarrhea continues.
Example
A stool sample with sodium 30 mmol/L and potassium 20 mmol/L:
- Electrolyte osmolality: 2 × (30 + 20) = 100 mOsm/kg
- Gap: 290 − 100 = 190 mOsm/kg, a clear osmotic pattern
Checking the sample
- A measured stool osmolality well below 250 suggests the sample was diluted with water or urine, sometimes deliberately in factitious diarrhea.
- A value well above 400 usually means bacterial fermentation continued after collection; the sample should reach the lab quickly or be refrigerated.
- Stool magnesium above about 45 mmol/L points to magnesium-induced diarrhea, and a low stool pH below 5.5 points to carbohydrate malabsorption.
Limits
The test needs liquid stool, so it does not apply to formed stools. Results guide further testing, such as celiac serology, stool calprotectin, colonoscopy with biopsies or a lactose breath test, rather than giving a final diagnosis. See a doctor for diarrhea with blood, weight loss, fever or night symptoms.