Breath testing for small intestinal bacterial overgrowth (SIBO) has been with us for many years but has faced a varied press from the medical community in recent times. In one camp, there has been negative press suggesting that the technique is inaccurate, over estimates the presence of SIBO and leads to inappropriate treatment. In a second camp, there has been large scale randomised controlled trials showing that breath testing can play an important role in identifying the underlying pathophysiology of patients with IBS like symptoms and that treatment of these symptoms with anti-biotics can be very successful. As usual with most contentious scientific / medical issues, the truth likely sits somewhere in between.

Having performed and analysed several thousand breath tests over the last few years and read the literature with an open mind, I have begun to form my own clinical opinions as to where breath testing fits in a modern 21st century Gastro-intestinal Physiology service. As with all tests, breath testing has strengths, weaknesses, physiological caveats and requires the physiologist or clinician to engage their brain and use clinical judgement when interpreting results that are often not black and white. The following piece is not intended to be a review of the literature, rather it discusses these issues generally and how I have interpreted these to form my own pragmatic (not dogmatic) position to generate further discussion within the clinical and patient communities.


– Hydrogen and methane gas production as a by-product of bacterial fermentation of an ingested substrate and underlies the principles of breath testing in the context of SIBO. These gases are absorbed into the bloodstream via the gut and excreted in the breath via the lungs. A rise in gas levels represents a physiological phenomenon which can be easily observed using a relatively un-invasive and non-cumulative technique. The clinical translation of these physiological findings can be increased by carefully recording symptoms, bowel movements, abdominal distension and the reproduction of the patient’s usual or most troublesome symptoms during or indeed sometimes after the test.

Glucose has been lauded as the most sensitive substrate to use as there is supposed to virtually no bacteria in the first few feet of small intestine where glucose is normally fully absorbed. Therefore a positive result means that proximal duodenal SIBO is present with certainty in the majority of cases. However, there are several proposed causes of SIBO such as PPI usage, previous gastroenteritis, abnormal motility, previous radiotherapy or surgery. Therefore one must design the protocol for the test to answer the specific question posed by the referring clinician and in the context of the patient’s symptom profile and clinical history. If a glucose breath test is normal, this has ruled out one type of SIBO but is only one step in the diagnostic pathway.

Lactulose is a non-digestible carbohydrate which is not absorbed in the small bowel but is fermentable by bacteria which breaks the substrate down to form short chain fatty acids (SCFA) gases and other by products. The main advantage of using lactulose for breath testing is that if there are bacteria in the gut, it will interact with the organisms, gases will be produced and be detected in virtually all cases. Therefore, a negative test (the parameters of which are described below) can be accepted with more certainty. The major criticism of lactulose breath testing is that the peaks relate to the speed of oro-caecal transit and if this is rapid then a false positive result could be interpreted and if it’s too slow then it makes interpretation more difficult.

Provocation test for bowel

In summary, our pragmatic approach to breath testing has yielded excellent results as demonstrated with growing partnerships with our referring clinicians, surgeons and dietitians. SIBO breath testing should be seen as a provocation tests which correlates clinical, physiological and symptomatic data to identify underlying pathophysiology.

Once the underlying pathophysiology has been identified and time spent with the patient explaining the impact that SIBO or caecal mal-fermentation can have on symptoms, the diagnostic pathway can lead on to the treatment pathway. Treatment can be difficult and again there is little consensus in the UK for the best way forward, but good, effective treatments do exist and a judicially designed breath test service can help to target those treatment strategies as well as objectively assess their effectiveness.

What happens when you have a breath test? 

Watch our short video Emma visits The Functional Gut Clinic -HERE- to see exactly what you can expect.