Laboratory Test Reference Guide

Osmotic gap < 50 suggest secretory diarrhoea

Osmotic gap > 125 suggest osmotic diarrhoea

Feacal osmolar gap = 290 - 2 x (Na +K). 

The test can only be performed if the faeces is watery, Sodium and Potassium is performed on the faeces. Faecal osmolality is not measured. The faecal osmolality starts to increase while still in the colon and keeps increasing after defaecation due to bacterial fermentation of unabsorbed carbohydrates. Presence of faecal reducing substances  AND a faecal pH of < 6 is suggestive of carbohydrate malabsorption as the cause of the osmotic diarrhoea.

Measured faecal osmolality should not be used. The osmolarity in faeces increase while inside the colon continues after defaecation, presumably due to continued fermentation of unabsorbed carbohydrates.

Faecal osmolar gap is calculated: 290 – 2 x (Sodium + Potassium). The gap is small (<50) in secretory diarrhoea and large (> 125 ) in osmotic diarrhoea.

A faecal pH of < 6  and positive test for faecal reducing substances suggests carbohydrate malabsorption as causing the osmotic diarrhoea.

Faecal osmolality may be of value to check whether a sample has been contaminated with water, such samples have an osmolality of < 290 mmol/kg.