GI ToolsGastroenterology and hepatology

Stool osmotic gap

The faecal osmotic gap separates secretory from osmotic diarrhoea. It uses 290 mOsm/kg as the stool osmolality, because stool osmolality rises after collection as bacteria ferment carbohydrate.

Stool results (liquid stool)

Enter the values above.

How it is worked out

Gap = 290 − 2 × (stool sodium + stool potassium).

References

Eherer AJ, Fordtran JS. Fecal osmotic gap and pH in experimental diarrhea of various causes. Gastroenterology. 1992;103(2):545-551.

Arasaradnam RP, Brown S, Forbes A, et al. Guidelines for the investigation of chronic diarrhoea in adults: British Society of Gastroenterology, 3rd edition. Gut. 2018;67(8):1380-1399.