What it is
The stool osmotic gap estimates how much of the stool’s osmolality is unaccounted for by its measured electrolytes. The logic: stool is iso-osmotic with plasma (~290 mOsm/kg), and its electrolyte-driven osmolality is the measured cations (Na + K) plus their accompanying anions — captured by doubling the cation sum. A large leftover gap means some other osmotically active, poorly absorbed solute (an unabsorbed sugar, a laxative) is driving water into the lumen — osmotic diarrhoea. A small gap means the diarrhoea is driven by electrolyte secretion itself — secretory diarrhoea.
The formula
Stool osmotic gap (mOsm/kg) = 290 − 2 × (stool Na + stool K)290 is the assumed normal stool osmolality. If a measured stool osmolality is available, use it in place of 290 (and ideally measure it promptly — see pitfalls).
Interpreting the gap
- > 100 mOsm/kg — osmotic diarrhoea. An unabsorbed solute is present: lactose / disaccharidase deficiency, sorbitol or other sugar alcohols, osmotic laxatives (Mg, lactulose, PEG), malabsorption. Characteristically resolves with fasting.
- < 50 mOsm/kg — secretory diarrhoea. Driven by active ion secretion: enterotoxins, congenital chloride/sodium diarrhoea, neuroendocrine tumours, bile-acid malabsorption. Characteristically persists despite fasting.
- 50–100 mOsm/kg — indeterminate / overlap; interpret with the fasting response and the clinical picture.
When to use it
Use it in the work-up of chronic or unexplained watery diarrhoea, when separating an osmotic from a secretory mechanism will change the differential and the next tests. Send stool for electrolytes (Na, K) and, where possible, osmolality. The bedside complement is a fasting trial: osmotic diarrhoea improves, secretory does not.
Worked example
Chronic watery diarrhoea, stool Na 30 mmol/L, stool K 20 mmol/L: gap = 290 − 2 × (30 + 20) = 290 − 100 = 190 mOsm/kg → >100, osmotic — pursue malabsorption / unabsorbed solutes, expect improvement on fasting.
Contrast: stool Na 110 mmol/L, stool K 30 mmol/L: gap = 290 − 2 × (110 + 30) = 290 − 280 = 10 mOsm/kg → <50, secretory — pursue toxins, congenital chloride diarrhoea, secretagogues; it will persist when the child is NPO.
Pitfalls and caveats
- Stool must be liquid. The gap is meaningless on formed stool; sample watery output.
- Don’t measure stool osmolality late — bacterial fermentation in a standing sample falsely raises it; an osmolality well below ~290 implies dilution with water or urine (and a measured value markedly above serum suggests fermentation or contamination).
- A very low stool osmolality (<250) points to factitious dilution (added water) rather than a true gap.
- The cut-offs are guides, not absolutes — the indeterminate 50–100 band is real, and mixed mechanisms occur. Combine with stool pH (low pH favours carbohydrate malabsorption), reducing substances, and the fasting response.
Run it: Stool Osmotic Gap
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.