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Stool Osmotic Gap

Stool osmotic gap = 290 − 2 × (stool Na + stool K). A gap >125 mOsm/kg suggests osmotic diarrhoea (e.g. magnesium, lactose, sorbitol); <50 mOsm/kg suggests secretory diarrhoea.

Stool sodium (mmol/L or mEq/L)
Stool potassium (mmol/L or mEq/L)

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Interpretation

Result	Meaning
< 50	Secretory
50–125	Indeterminate
> 125	Osmotic

Formula

Stool osmotic gap = 290 − 2 × (stool Na⁺ + stool K⁺)

Clinical pearls

Uses 290 mOsm/kg (plasma osmolality) rather than measured stool osmolality, which rises with bacterial fermentation after collection.
A measured stool osmolality well below 290 implies the sample was diluted with water or urine (factitious diarrhoea).
Some sources use >100 as the osmotic threshold; 125 is from Eherer & Fordtran.

Examples

stool osmotic gap stool na 30 stool k 40 fecal osmotic gap with stool sodium 100 and potassium 40

FAQ

What is a normal stool osmotic gap?

There is no single normal; the gap is used only in diarrhoea, where <50 points to secretory and >125 to osmotic causes.

Why is 290 used instead of measured stool osmolality?

Stool in the colon is iso-osmolar with plasma; measured osmolality rises after collection from bacterial fermentation, so 290 is more accurate.

Evidence

Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology 1992;103:545–51.

Hepatology & GI · Reviewed 2026-10-14 · Methodology