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General MedicineStool Osmotic Gap

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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mEq/L
mEq/L

Use sodium and potassium from the same properly collected liquid stool specimen or report. Do not enter serum or urine sodium or potassium. For monovalent Na⁺ and K⁺, mEq/L and mmol/L have the same numeric value.

Stool osmotic gap will appear here

Enter stool sodium and potassium from the same properly collected stool specimen.

The calculator uses the conventional assumed 290 mOsm/kg model and reports an estimate without assigning a diarrhea diagnosis.

About

This calculator estimates the stool osmotic gap from sodium and potassium measured in the same properly collected liquid stool specimen. It is a physiologic clue used in selected evaluations of chronic or otherwise unexplained watery diarrhea—not a diagnosis of a diarrhea mechanism, cause, or treatment. [1, 10, 11]

The model assumes 290 mOsm/kg rather than using measured stool osmolality. Positive, zero, and negative arithmetic results are retained without automatic classification.

Formula

Estimated stool osmotic gap = 290 − 2 × (stool sodium + stool potassium), reported in mOsm/kg. [1, 10, 11]
Sodium and potassium are monovalent, so their mEq/L and mmol/L values are numerically equivalent. The factor of 2 accounts for their accompanying anions in the calculated electrolyte contribution. [7, 10]
The 290 mOsm/kg term is a conventional model constant, not a direct measurement of this specimen. [1, 4, 10]

Interpretation

What is the stool osmotic gap?

The estimate describes the portion of the conventional 290 mOsm/kg stool-water model not accounted for by sodium, potassium, and their accompanying anions. In a selected chronic watery diarrhea work-up, it can provide one physiologic clue about electrolyte-driven versus poorly absorbed solute contributions. It is neither a direct measurement of unidentified solutes nor a disease diagnosis. [1, 7, 10]

How to calculate the stool osmotic gap

Subtract twice the sum of stool sodium and potassium from 290: 290 − 2 × (stool Na + stool K). The factor of 2 represents the accompanying anions that contribute alongside the measured cations. Use both electrolytes from one properly collected liquid stool specimen or report. [1, 10, 11]

Example 1

Stool Na 30.0 mEq/L; stool K 20.0 mEq/L

190.0 mOsm/kg

290.0 − 2 × (30.0 + 20.0) ≈ 190.0 mOsm/kg

Example 2

Stool Na 100.0 mEq/L; stool K 45.0 mEq/L

0.0 mOsm/kg

290.0 − 2 × (100.0 + 45.0) ≈ 0.0 mOsm/kg

These examples illustrate arithmetic only and do not assign a diarrhea mechanism.

Why does the formula use 290 mOsm/kg?

The conventional model assumes fresh intestinal or stool water is approximately equilibrated with body-fluid osmolality and fixes the starting term at 290 mOsm/kg. It is therefore a model constant—not a patient input or a measured osmolality from the submitted specimen. [1, 4, 10]

What does a low stool osmotic gap mean?

Several sources use <50 mOsm/kg as a low-gap, electrolyte-transport, or secretory-pattern reference. That is not a diagnosis: in Eherer and Fordtran's experimental study, sodium-sulfate-induced osmotic diarrhea could also produce a gap below 50. The calculator therefore does not apply the boundary dynamically. [1, 7, 11]

What does a high stool osmotic gap mean?

A higher gap means measured sodium and potassium account for a smaller share of the assumed 290 mOsm/kg model. That pattern may occur when poorly absorbed or non-electrolyte osmoles contribute, but the high-end boundary varies by source and clinical context. A number alone does not identify the solute or establish one mechanism. [2, 3, 7, 10]

Is there a normal or intermediate stool osmotic gap?

Some references describe 50–100 or 50–125 mOsm/kg as an intermediate or overlap range. These are not universal healthy reference intervals. Mixed mechanisms, study methods, specimens, and patient populations differ, so this page assigns no green “normal” result and no automatic category. [3, 7, 8]

Source-specific cutoff context

Published value
<50 mOsm/kg
Source-specific context
A commonly cited low-gap reference in experimental and clinical summaries; exceptions occur.
Published value
>75 mOsm/kg
Source-specific context
Used as a high-gap reference in the 2017 fecal-electrolyte laboratory review.
Published value
>100 mOsm/kg
Source-specific context
Used in some reviews and pediatric frameworks for a higher-gap pattern.
Published value
>125 mOsm/kg
Source-specific context
Used by other clinical references for a high-gap pattern.
Published value
>160 mOsm/kg
Source-specific context
The clear high-gap observation in Shiau et al.'s 1985 twelve-patient series.

These values are source-specific evidence context, not five calculator bands. The result card does not compare the submitted value with any of them. [1, 2, 3, 7, 8, 9, 10]

Why specimen collection and handling matter

Fecal electrolytes are intended for an appropriate liquid stool specimen. Collection, timing, storage, dilution, water or urine contamination, and the laboratory's assay validation can affect interpretation. Fecal fluid is a nonstandard laboratory matrix, and the 2017 validation study emphasized local analytical validation rather than assuming any chemistry assay is suitable. [2, 4, 10]

Room-temperature storage and bacterial fermentation can artifactually raise measured stool osmolality. Duncan et al. also found technical and discriminatory problems when measured stool osmolality was used in the gap calculation. Direct measurement may instead help investigate contamination, adulteration, or sample integrity; it is not an input replacing 290 in this fixed-model calculator. [2, 4, 6, 10]

Stool osmotic gap vs serum osmolar gap

This stool calculation uses stool sodium, stool potassium, and a fixed 290 model in watery-diarrhea physiology. A serum osmolar—or osmolal—gap uses a different specimen, analytes, and clinical task. The related serum osmolality calculator is a separate tool; its values are not interchangeable with this one.

Stool osmotic gap, osmolal gap, and osmolar gap

Clinical sources use stool osmotic gap, stool osmolality gap, stool osmolal gap, and stool osmolar gap with some overlap. LOINC 73571-2 is the active “Osmol gap of Stool” term. This page uses “stool osmotic gap” consistently as the product name and does not create separate equations for the synonyms. [12]

Clinical boundaries

Interpret the estimate with symptoms, duration, stool volume, fasting response, medicines or laxatives, diet, sample quality, and other testing. A negative value remains a mathematical result and should prompt specimen, method, unit, and context review—not automatic clamping or diagnosis. This calculator does not diagnose osmotic, secretory, inflammatory, fatty, or another diarrhea mechanism and does not determine treatment. [2, 5, 6, 11]

References

  1. Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103(2):545–551. PMID 1634072. DOI 10.1016/0016-5085(92)90845-P.
  2. 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.
  3. Shiau YF, Dharmsathaphorn K. Clinical and laboratory approaches to evaluate diarrheal disorders. Crit Rev Clin Lab Sci. 1987;25(1):43–69. PMID 3301211. DOI 10.3109/10408368709105877.
  4. Duncan A, Robertson C, Russell RI. The fecal osmotic gap: technical aspects regarding its calculation. J Lab Clin Med. 1992;119(4):359–363. PMID 1583385.
  5. Binder HJ. The gastroenterologist's osmotic gap: fact or fiction? Gastroenterology. 1992;103(2):702–704. PMID 1634087. DOI 10.1016/0016-5085(92)90870-5.
  6. Schiller LR, Pardi DS, Spiller R, et al. Gastro 2013 APDW/WCOG Shanghai Working Party Report: chronic diarrhea: definition, classification, diagnosis. J Gastroenterol Hepatol. 2014;29(1):6–25. DOI 10.1111/jgh.12392.
  7. Semrad CE. Approach to the Patient with Diarrhea and Malabsorption. In: Goldman's Cecil Medicine. 24th ed. 2012:895–913. PMCID PMC7152045. DOI 10.1016/B978-1-4377-1604-7.00142-1.
  8. Thiagarajah JR, Kamin DS, Acra S, et al. Advances in Evaluation of Chronic Diarrhea in Infants. Gastroenterology. 2018;154(8):2045–2059.e6. PMCID PMC6044208. DOI 10.1053/j.gastro.2018.03.067.
  9. Castro-Rodríguez JA, Salazar-Lindo E, León-Barúa R. Differentiation of osmotic and secretory diarrhoea by stool carbohydrate and osmolar gap measurements. Arch Dis Child. 1997;77(3):201–205. PMID 9370895. PMCID PMC1717323. DOI 10.1136/adc.77.3.201.
  10. Parakh R, Greene DN, Mathias PC, Block DR, Ranjitkar P. Laboratory Utilization and Analytical Validation of Fecal Electrolyte Tests. J Appl Lab Med. 2017;1(6):668–677. DOI 10.1373/jalm.2016.022590.
  11. Gotfried J. Diarrhea. Merck Manual Professional Edition. Reviewed/Revised May 2024; modified April 2025. Accessed September 4, 2026.
  12. Regenstrief Institute. LOINC 73571-2: Osmol gap of Stool. Active terminology record. Accessed September 4, 2026.

FAQ

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Disclaimer

Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.