stomach pain

Lactulose breath test for a SIBO diagnosis: is it accurate?

Susan Hunter Digestive Health Leave a Comment

Have you been researching the possible causes of your digestive problems and suspect you have small intestinal bacterial overgrowth (SIBO)? Are you unsure how to investigate this further?

If so, you’ve found the right article.

The gold standard test for diagnosing SIBO is what’s known as a ‘jejunal aspirate and culture’. It accurately diagnoses SIBO in the duodenum and upper jejunum but it’s an invasive, expensive and impractical test that is not commonly ordered by health practitioners.

gut healthThe most popular test used to diagnose SIBO is the lactulose and/or glucose breath test. Bacteria in the small and large intestine produce hydrogen or methane when they digest and ferment unabsorbed carbohydrates (read more about that here). When a lactulose or glucose solution is ingested, the hydrogen or methane produced determines whether SIBO is present.

stomach painBreath testing has its limitations. The glucose test accurately detects SIBO in the proximal ileum (the beginning of the ileum) but not the distal ileum (the farthest part of the small intestine that joins the colon/bowel). This is because glucose is mostly absorbed in the duodenum (the part of the small intestine that is attached to the stomach), so there’s little sugar left for the bacteria to ferment further down the tract.

In order to understand if there is SIBO in the distal ileum it is widely thought that we can do the lactulose breath test.

The lactulose breath test was originally used as a test for oral-cecal transit time or as a preliminary test that is done to determine whether a person is a hydrogen or methane breather when testing for fructose malabsorption. Most individuals breath out hydrogen after consuming lactulose, while others (mostly people that are constipation predominant) will breath methane.

Lactulose is malabsorbed by us but bacteria quickly use it as a fuel source. When lactulose is being absorbed by bacteria in large numbers then a SIBO diagnosis can be made (more on that here). If there is a rise of more than 20 parts per million over a 20 minute period in the first 90 minutes of testing for hydrogen breathers and a rise of more than three parts per million in methane breathers, then SIBO is present.


This diagnostic criteria is based on the presumption that the mouth to cecum transit time is always greater than 90 minutes, so a peak in breath hydrogen within the 90 minute time frame must be due to bacteria fermenting lactulose in the small intestine.

The concern with this assessment is that transit time of the gut varies across ethnicities. Asian and Indian people tend to have a faster transit time, so they malabsorb lactulose faster and in higher quantities and would be diagnosed with SIBO based on this criteria when they do not have it. The average transit time for Indian people was found to be 65 minutes and 85 minutes in the Taiwanese population. This is also the case with people who experience anxiety. They also tend to have a faster transit time and this would mean a false positive SIBO result too (more here).

A word of warning, however.

The accuracy of the lactulose breath test for SIBO diagnosis has to be questioned when we cannot be certain that the high hydrogen or methane reading is being caused by malabsorption of lactulose in the small intestine. It could actually be malabsorption in the large bowel creating methane or hydrogen which is a normal process.

Susan Hunter
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Susan Hunter

B.Hsc (Naturopathy), B.A. (Psych) at Healthful Clinic - Founder and Director
Susan Hunter is a double-degree qualified naturopath, a published academic author and the founder of Healthful Clinic. She writes widely on mental health, digestive conditions and children's health.
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