Stool Osmotic Gap
Differentiates osmotic from secretory diarrhoea. Osmotic gap >125 mOsm/kg suggests osmotic diarrhoea (stops with fasting). Gap <50 mOsm/kg suggests secretory diarrhoea (continues with fasting).
Score interpretation
Gap <50 mOsm/kg — secretory diarrhoea. Stool electrolytes account for most of osmolality.
→ Investigate for: infections (cholera, VIPoma), neuroendocrine tumours, bile acid malabsorption, microscopic colitis, medication-induced; diarrhoea persists with fasting
Gap 50–125 mOsm/kg — indeterminate or mixed
→ Consider mixed picture; assess clinical context; fasting test may help differentiate
Gap >125 mOsm/kg — osmotic diarrhoea. Non-absorbable solutes retaining water in bowel.
→ Investigate for: lactase deficiency, sorbitol/lactulose use, malabsorption, Mg2+-containing laxatives; diarrhoea stops with fasting
Interpretation bands for the Stool Osmotic Gap. Apply clinical judgement and local guidance.
References
- Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103(2):545–551.
Related
Curated clinical cross-links plus same-class fallbacks.
- Mannitol · Osmotic diuretic
- Macrogol (Polyethylene Glycol) · Osmotic Laxative
- Lactulose · Osmotic laxative / Ammonia reduction (hepatic encephalopathy)
- Lactulose (Hepatic Encephalopathy) · Osmotic Laxative / Ammonia-Reducing Agent
- Macrogol (Polyethylene Glycol) · Osmotic Laxative
- Sodium Acid Phosphate Enema · Osmotic / Stimulant Rectal Laxative
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.