Stool Osmotic Gap Calculator

Calculate stool osmotic (osmolar) gap from measured stool osmolality and stool sodium and potassium. Learn typical secretory vs osmotic pattern bands, specimen issues, and limitations for chronic watery diarrhea.

Stool Osmotic Gap Calculator

Measured stool osmolality (mOsm/kg)

From stool water / supernatant by osmometer

Stool sodium (Na⁺) (mmol/L or mEq/L)

Stool potassium (K⁺) (mmol/L or mEq/L)

Disclaimer: This tool is for education and decision support only. Stool electrolyte and osmolality testing is not always available; specimen collection and dilution affect results. Do not use the gap alone to diagnose or treat—integrate with history, examination, and guideline-based evaluation.

Stool osmotic (osmolar) gap

Formula

Stool osmotic gap = measured stool osmolality − 2 × (Na⁺_stool + K⁺_stool)

Units:

Measured stool osmolality — mOsm/kg (or mOsm/L for dilute aqueous stool water; clinically treated comparably in this calculation).
Stool Na⁺ and K⁺ — mmol/L (same numeric value as mEq/L for these monovalent ions).

The term 2 × (Na⁺ + K⁺) estimates the osmotic contribution of those measured cations (with anions assumed to balance), so the gap reflects osmoles not explained by stool sodium and potassium—often unmeasured solutes in osmotic diarrhea, versus stool that is closer to an electrolyte-rich, plasma-like fluid in many secretory states.

Common interpretive cutoffs

Cutoffs are teaching conventions, not pathognomonic thresholds. Always interpret with clinical context, medications, and whether the sample truly represents watery stool supernatant.

Gap < 50 mOsm/kg

Often described as a secretory pattern: measured tonicity is largely accounted for by Na⁺ and K⁺.

50–125 mOsm/kg

Gray zone—mixed mechanisms, partial treatment, or technical factors are common.

Gap > 125 mOsm/kg

Often described as an osmotic pattern: substantial unmeasured osmoles (e.g., laxatives, poorly absorbed carbohydrates).

Limitations

• Not all labs offer stool osmolality; collection method and dilution change results.
• After some osmotic agents, timing of the sample relative to ingestion matters.
• The gap does not replace evaluation for infection, inflammation, malabsorption, or surgical causes.
• Pediatric and adult reference patterns still require clinician judgment.

Overview

The stool osmotic gap (sometimes called the stool osmolar gap) is a simple arithmetic construct used mainly in the evaluation of chronic or persistent watery diarrhea. It combines a measured stool osmolality with stool sodium and potassium concentrations to estimate how much of the stool’s tonicity is explained by those electrolytes versus unmeasured osmotically active solutes. It is an adjunct to history, examination, and targeted testing—not a stand-alone diagnosis.

Why the gap exists

Stool water contains many dissolved particles that contribute to osmotic activity. In clinical practice, laboratories often measure stool (or stool supernatant) osmolality with an osmometer. They may also measure Na⁺ and K⁺ in the same fluid. Monovalent ions such as sodium and potassium each contribute roughly one osmole per mole in solution; together with obligate anions that maintain electroneutrality, the electrolyte framework is commonly summarized for teaching purposes as:

Electrolyte-related contribution ≈ 2 × ([Na⁺] + [K⁺])

Here, sodium and potassium are expressed in mmol/L (numerically the same as mEq/L for these ions). The factor of two reflects the paired cation–anion effect as it is typically taught for this bedside calculation.

Anything that raises measured stool osmolality without being captured in that electrolyte estimate will widen the gap. Common examples include poorly absorbed carbohydrates, certain laxative osmoles, and some sugar alcohols—substances that may not be reflected as a large rise in measured Na⁺ and K⁺ in the same specimen.

Formula used in this calculator

The stool osmotic gap is calculated as:

Stool osmotic gap = measured stool osmolality − 2 × (stool [Na⁺] + stool [K⁺])

Measured stool osmolality is reported in mOsm/kg (or institutionally as mOsm/L for dilute aqueous stool water; the arithmetic is applied the same way in common clinical teaching).
Stool [Na⁺] and [K⁺] are in mmol/L (mEq/L).

The calculator also displays 2 × (Na⁺ + K⁺) so you can see how much of the measured osmolality is attributed to the electrolyte model before subtracting to obtain the gap.

Interpretive bands (teaching cutoffs)

Textbooks and reviews often stratify results into three regions. These thresholds are heuristic, not pathognomonic, and there is biological and technical overlap.

Low gap (commonly < 50 mOsm/kg): Often described as a pattern more consistent with secretory-type physiology in didactic frameworks: measured tonicity is largely explained by stool sodium and potassium, suggesting stool fluid that is relatively electrolyte-rich and closer to an “isotonic” profile. Many secretory etiologies are discussed in this context, but infection, post-surgical anatomy, medications, and endocrine diarrhea can also produce overlapping electrolyte patterns—so the gap must be integrated with the rest of the workup.
High gap (commonly > 125 mOsm/kg): Often described as more consistent with an osmotic component: a substantial fraction of stool tonicity comes from solutes not represented in the Na⁺/K⁺ estimate. This pattern prompts consideration of ingested or poorly absorbed osmoles (for example, some laxatives, non-absorbed carbohydrates, sorbitol or related compounds, and malabsorptive states), always correlated with diet, medications, and timing of sampling.
Intermediate gap (often ~50–125 mOsm/kg): Frequently labeled an indeterminate or overlap zone. Mixed mechanisms, partial treatment, variable hydration of the specimen, or laboratory handling issues commonly land patients here.

When clinicians find the gap useful

The stool osmotic gap is most informative when the question is whether a watery diarrhea syndrome is likely dominated by electrolyte secretory losses versus osmotically driven water movement due to luminal solutes. It can help structure the differential in selected patients with unexplained chronic diarrhea, especially when coupled with:

A careful medication review (including over-the-counter agents, supplements, and magnesium-containing products)
Diet history (lactose and other carbohydrates, “diet” products, sugar alcohols)
Surgical anatomy and prior bowel resections
Targeted stool studies and endoscopic evaluation when indicated

It is generally less helpful when diarrhea is clearly explained by an identified infection or inflammatory process, or when stool studies are not performed in a standardized way.

Specimen and laboratory considerations

Results are only as reliable as the specimen. Important practical points include:

Collection and supernatant: Laboratories differ in whether they analyze whole stool, filtrates, or supernatant after centrifugation. Inconsistent preparation changes electrolyte and osmolality relationships.
Dilution and water content: Very watery stool may still yield plausible numbers, but mishandling (dilution with water, toilet water contamination) can invalidate interpretation.
Timing relative to ingestion: For osmotic agents, the gap may vary with proximity to the offending meal or dose.
Availability: Not all centers routinely offer stool osmolality; when unavailable, clinicians rely more heavily on history, empiric trials, and other tests.

Limitations and pitfalls

The gap is a model, not a direct measurement of a single physiologic variable.
It does not replace evaluation for infection, inflammation, ischemia, motility disorders, or malabsorption syndromes when those are clinically suspected.
Overlap is expected; borderline values should not be over-interpreted.
Pediatric populations, enteral feeds, IV fluids, and renal disorders can alter stool electrolytes in ways that change the gap without mapping neatly onto adult textbook examples.
Always interpret alongside serum electrolytes, volume status, and the overall clinical trajectory.

Using this calculator responsibly

Enter the measured stool osmolality and the stool sodium and potassium from the same clinical context and, when possible, the same laboratory handling pathway. Use the output to organize reasoning and discussion with a treating clinician rather than as a definitive label. Any persistent diarrhea associated with dehydration, weight loss, bleeding, fever, or alarm features warrants in-person evaluation and guideline-directed testing.