Stool Osmotic Gap Calculator
Estimate stool osmotic gap from stool sodium and potassium using the conventional assumed 290 mOsm/kg model, with collection and interpretation limits.
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Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.
About
Formula
Interpretation
Commonly cited stool osmotic-gap context
| Commonly cited context | Educational physiologic example |
|---|---|
| Below 50 mOsm/kg | Often described as a low-gap or electrolyte-transport/secretory-pattern example. |
| 50–125 mOsm/kg | Intermediate range that may not clearly distinguish one mechanism. |
| Above 125 mOsm/kg | Often described as a high-gap or osmotic-pattern example. |
These are commonly cited examples, not universal diagnostic thresholds. Published studies and reviews have used different cutoffs; intermediate and mixed mechanisms occur. The calculator never compares or highlights a submitted result. A low gap does not diagnose secretory diarrhea, a high gap does not diagnose osmotic diarrhea, and the result does not identify one cause or treatment.
Assumed versus measured osmolality
This page keeps the conventional assumed 290 mOsm/kg in the formula. Directly measured stool osmolality addresses different preanalytic questions: collection delay, bacterial fermentation during storage, and urine or water contamination can affect a measured value. A measured value should not replace the fixed 290 in this calculator.
A negative gap is a retained arithmetic result, not an automatic zero. Recheck sample collection, storage, dilution or contamination, units, and consistency of the results; do not use a negative value to assign one diarrhea mechanism.
Interpret any estimate alongside history, fasting response, medicines or laxatives, diet, stool volume, sample quality, and other testing. This page does not diagnose osmotic, secretory, inflammatory, fatty, or other diarrhea mechanisms and does not select treatment.
References
- Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103:545-551. PMID 1634072. DOI 10.1016/0016-5085(92)90845-P.
- Shiau YF, Feldman GM, Resnick MA, Coff PM. Stool electrolyte and osmolality measurements in the evaluation of diarrheal disorders. Ann Intern Med. 1985;102(6):773–775. PMID 3994188. DOI 10.7326/0003-4819-102-6-773.
- Shiau YF. Clinical and laboratory approaches to evaluate diarrheal disorders. PMID 3301211. DOI 10.3109/10408368709105877.
- Binder HJ. The gastroenterologist's osmotic gap: fact or fiction? Gastroenterology. 1992. PMID 1634087. DOI 10.1016/0016-5085(92)90870-5.
- Approach to the Patient with Diarrhea and Malabsorption. PMCID PMC7152045.
- Advances in Evaluation of Chronic Diarrhea in Infants. PMCID PMC6044208.
- Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements. PMID 9370895.
FAQ
This page calculates 290 minus twice the sum of stool sodium and stool potassium. It reproduces the conventional assumed-290 arithmetic only.
No. 290 mOsm/kg is an assumed stool-water osmolality in this calculation, not a measured osmolality from the submitted sample.
Enter both electrolyte values in mEq/L. For monovalent sodium and potassium, mEq/L and mmol/L have the same numeric value.
Yes. Zero is valid mathematical input for either electrolyte, provided the reported result and units are correct.
The gap is zero when twice the sum of stool sodium and potassium equals 290, meaning 2 × (sodium + potassium) = 290. It can be negative when that contribution exceeds 290; this page retains the arithmetic rather than clamping it.
Reviews often describe low-gap and high-gap physiologic patterns, but published cutoffs differ. These historical examples do not establish one mechanism in an individual sample.
No. Studies and reviews have used different cutoffs, and intermediate or mixed mechanisms occur.
No. A single estimated gap cannot diagnose or exclude osmotic, secretory, inflammatory, fatty, or other diarrhea mechanisms.
An intermediate result may not clearly distinguish one mechanism and should be interpreted with the full clinical and laboratory context.
Dilution, delayed processing, storage, bacterial fermentation, and urine or water contamination can affect stool-related interpretation.
Yes. Contamination or dilution can make electrolyte and osmolality-related results difficult to interpret and should prompt review of sample quality.
Direct measurement can address separate preanalytic questions such as storage-related fermentation or contamination. It should not replace the fixed 290 used in this calculator.
Yes. These contextual factors may inform an evaluation of diarrhea but are not inputs to this arithmetic calculator.
No. This page does not determine hydration, diet, medication changes, antibiotics, antidiarrheal therapy, or any other treatment.
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Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.