What is a stool reducing substances test?

Stool (or faecal) reducing substances is a stool sample test used to diagnose lactose intolerance (and some rare metabolic abnormalities). Lactose intolerance can be caused by a prolonged or severe episode of viral gastroenteritis.

How is a stool reducing substances test performed?

The faecal reducing substances test is performed in a laboratory, on a sample of stool as small as 5 grams.
Unfortunately this sample needs to be delivered to the laboratory as soon as possible, preferably within 1 hour. This is because lactose (or other sugars) in the stool will normally be broken down by chemical processes within 2-4 hours afetr the specimen is produced.

Why would you need a stool reducing substances test?

Faecal reducing substances may be ordered by your doctor for symptoms suggestive of lactose intolerance, for example frequent loose stools, particularly if they are frothy in nature.

Lactose intolerance may occur after a prolonged episode of viral gastroenteritis, due to inadequate absorption of the sugar lactose by a damaged intestinal lining.

Other conditions, in which other sugars, such as glucose, galactose and fructose are not absorbed properly, can also cause a positive test for reducing sugars in the stool.

Test results, explained

Faecal reducing substances are reported as:

• Negative – this is the normal result and means that the body is digestying and absorbing sugars properly
• Positive – this means there are substances in the stool that can act as ‘reducing agents’, i.e. there are forms of sugar in the stool that have not been absorbed by the body

Related specialists

• General Practitioner (GP)
• General Physician
• Gastroenterologist
• Infectious Diseases Physician
• Paediatrician
• Travel Medicine Physician

Related procedures

• Endoscopy of the Upper Gastrointestinal Tract
• Colonoscopy

Related tests

• Stool MCS (Microscopy, Culture and Sensitivity)
• Endoscopy of the Upper Gastrointestinal Tract
• Colonoscopy
• Urea & Electrolytes
• Liver Function Tests
• Full Blood Count

Also known as

• Faecal reducing substances
• Faecal reducing sugars

This article is for informational purposes only and should not be taken as medical advice. If in doubt, HealthEngine recommends consulting with a registered health practitioner.