What is the stool osmotic gap?

The stool osmotic gap (SOG) estimates the contribution of unmeasured, osmotically active solutes in stool. It is calculated as 290 − 2 × (stool Na + stool K), where 290 mOsm/kg represents the assumed stool osmolality.

What does a high osmotic gap mean?

A gap greater than 125 mOsm/kg indicates osmotic diarrhea. Common causes include carbohydrate malabsorption (e.g., lactose intolerance), osmotic laxative use (e.g., magnesium, lactulose), and celiac disease.

What does a low osmotic gap mean?

A gap below 50 mOsm/kg suggests secretory diarrhea, where the gut actively secretes electrolytes and water. Causes include microscopic colitis, VIPoma, bile acid malabsorption, and certain infections.

What is the normal or indeterminate range?

Values between 50 and 125 mOsm/kg are considered indeterminate. Further clinical history, imaging, and laboratory testing are needed to distinguish the type of diarrhea in this range.

Why is 290 used as the stool osmolality?

Stool is isotonic with plasma in the colon. Plasma osmolality is approximately 290 mOsm/kg, so this value is used as a fixed reference to represent total stool osmolality in the formula.

Is this calculator a substitute for medical advice?

No. This tool is intended for educational and informational purposes only. Always consult a qualified healthcare provider for diagnosis and treatment decisions.

Stool Osmotic Gap Calculator

Name: Stool Osmotic Gap Calculator
Availability: InStock
Author: Nham Vu

Enter stool sodium and potassium concentrations to calculate the fecal osmotic gap and classify diarrhea as osmotic or secretory.

Stool Electrolytes

Enter measured stool sodium and potassium concentrations.

Stool Sodium (Na⁺) mEq/L

Stool Potassium (K⁺) mEq/L

Formula

SOG = 290 − 2 × (Na + K)

Assumes stool osmolality = 290 mOsm/kg (plasma isotonicity)

Enter Na and K values,
then click Calculate.

Summary

Enter stool sodium and potassium concentrations to calculate the fecal osmotic gap and classify diarrhea as osmotic or secretory.

How it works

Collect a stool sample and measure sodium (Na) and potassium (K) concentrations in mEq/L.
Enter both values into the calculator fields.
The tool applies the standard formula: 290 − 2 × (Na + K).
The result is displayed in mOsm/kg with an interpretation badge.
Use the interpretation to guide further workup for osmotic or secretory causes.
Consult a physician — this calculator is for educational purposes only.

Use cases

Classify chronic diarrhea as osmotic or secretory during a GI workup.
Support clinical decision-making when evaluating malabsorption syndromes.
Help identify laxative abuse (osmotic pattern) in unexplained diarrhea.
Assist gastroenterology students in learning fecal electrolyte interpretation.
Quickly cross-check a hand-calculated osmotic gap for accuracy.
Screen for carbohydrate malabsorption or lactose intolerance patterns.
Aid in evaluating patients with inflammatory bowel disease flares.
Provide a rapid bedside reference for hospitalized patients with diarrhea.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu