SIBO Breath Test
A non-invasive breath test that measures hydrogen and methane gases to assess whether fermentation is occurring in the small intestine. Useful for persistent bloating, IBS-type symptoms, gas, discomfort after meals and unpredictable bowels.
What the SIBO breath test measures
SIBO occurs when bacteria that normally live in the colon migrate upwards into the small intestine, or when bacteria in the small intestine multiply beyond normal levels. When this happens, these bacteria ferment carbohydrates and produce hydrogen gas, which is absorbed into the bloodstream and eventually exhaled through the lungs.
Some people also have overgrowth of methane-producing archaea, a distinct group of microorganisms. This is now referred to as intestinal methanogen overgrowth (IMO) and is considered a separate condition from SIBO, although it causes similar symptoms. The breath test can detect both hydrogen and methane, helping to guide which treatment approach may be most appropriate.
After drinking a sugar solution (commonly lactulose or glucose), breath samples are collected at set intervals. If gas production rises sooner than expected, it may suggest that fermentation is occurring higher up in the digestive system than normal.
How the home breath test works
The SIBO breath test is typically done at home following clear instructions provided by the laboratory or clinic.
You fast overnight, usually for 12 hours, and avoid certain foods and medications for a period beforehand (antibiotics for four weeks, and laxatives or prokinetics for one week).
On the morning of the test, you provide a baseline breath sample by blowing into a collection tube or bag.
You then drink a solution containing either glucose or lactulose.
Over the next 90 to 180 minutes, you provide breath samples at regular intervals (usually every 15 to 20 minutes).
The samples are then sent to the laboratory for analysis of hydrogen and methane levels. Results are interpreted based on established criteria: a rise in hydrogen of 20 parts per million or more within 90 minutes is considered positive for SIBO, while elevated methane (above 10 parts per million at any point) suggests IMO. The test is safe, non-invasive and suitable for most people.
When a SIBO test might be worth considering
Breath testing for SIBO is most commonly considered when someone has persistent digestive symptoms that have not responded to initial dietary or lifestyle changes, particularly if those symptoms include:
Bloating and abdominal distension, especially after eating.
Reactions to certain carbohydrate or high-FODMAP foods.
Abdominal pain or discomfort.
Diarrhoea, constipation or alternating bowel patterns.
Excessive gas or flatulence.
Symptoms resembling irritable bowel syndrome (IBS) that have not improved with standard management.
A history of food poisoning, abdominal surgery or long-term antibiotic use.
SIBO is also more common in people with certain underlying conditions, such as structural abnormalities of the gut, low stomach acid, slow gut motility, immune system dysfunction or a history of abdominal surgery. If you have one of these risk factors and ongoing symptoms, testing may provide useful information to guide treatment.
Why routine NHS tests don't cover SIBO
GP testing focuses on identifying clear medical conditions that require treatment or referral, such as coeliac disease, inflammatory bowel disease or colorectal cancer. SIBO breath testing, by contrast, is a functional test that looks for patterns of microbial overgrowth, which do not always show up on routine blood tests, stool samples or endoscopies.
The most direct way to diagnose SIBO in a hospital setting is through upper endoscopy with aspiration of fluid from the small intestine, which is then cultured to count bacteria. However, this is invasive, costly, requires specialist equipment and is subject to sampling error (because bacteria are not evenly distributed throughout the small intestine), so it is rarely used outside of research or very specific clinical scenarios.
Breath testing is far more practical and widely available, which is why it has become the most common method for assessing SIBO in both NHS and private settings where it is offered.
However, it is not routinely available on the NHS in all areas, beacause NHS testing must prioritise diagnostics that change medical treatment. While SIBO may contribute to symptoms, its management is often dietary or lifestyle-driven - which falls outside the NHS model. This is why exploring SIBO often happens through nutrition professionals.
What your SIBO results can (and cannot) tell you
A positive SIBO breath test suggests that there is likely excessive bacterial fermentation occurring in the small intestine, which may be contributing to your symptoms. Elevated methane results indicate the presence of methane-producing archaea, which are linked to slower gut motility and constipation.
However, breath testing has important limitations:
Results can be affected by how quickly food moves through your gut (orocecal transit time), which varies between individuals.
False positives can occur, especially with lactulose testing, if the sugar reaches the colon before the test period ends.
False negatives are also possible, particularly if you have very slow gut motility or if bacteria present do not produce much hydrogen or methane.
Some researchers have raised concerns about over-diagnosis of SIBO using breath tests, arguing that the criteria may not be specific enough and that treatment should always be guided by symptoms and clinical context, not test results alone.
Despite these limitations, breath testing remains the most practical and non-invasive tool currently available for assessing SIBO, and it can be helpful in tailoring treatment, particularly when it comes to choosing the right type of antibiotic or herbal antimicrobial therapy.
How SIBO testing can guide nutrition support
If a SIBO breath test is positive, the focus of nutritional support is usually on reducing fermentation in the small intestine while addressing any underlying factors that may have contributed to bacterial overgrowth in the first place. This may include:
Dietary strategies to reduce foods that feed bacteria, such as a low FODMAP diet or specific carbohydrate diet, used temporarily and under guidance.
Supporting gut motility through adequate hydration, movement, stress management and, where appropriate, prokinetic nutrients or medications.
Addressing low stomach acid, if relevant, to help prevent bacteria from migrating upwards from the colon.
Working alongside medical treatment (antibiotics or herbal antimicrobials) to manage the overgrowth itself.
Gradually reintroducing a wider variety of foods once symptoms improve, to support long-term gut health and microbial diversity.
It is important to emphasise that SIBO is not usually a standalone condition. There is almost always an underlying reason why it has developed, and addressing that reason (for example, slow motility, structural issues or immune dysfunction) is just as important as treating the overgrowth itself.
When SIBO-type symptoms need GP review
You should speak with your GP if you notice:
unexplained weight loss
blood in the stool
black, tarry stools
severe or worsening abdominal pain
persistent vomiting
fever, night sweats or other signs of infection
symptoms that worsen quickly or feel significantly different from your usual pattern
If SIBO breath test results are positive, it is often helpful to share them with your GP, particularly if you have other medical conditions or are considering antibiotic treatment
Questions about SIBO?
If you’re dealing with ongoing bloating, gas or unpredictable bowels and aren’t sure what’s causing them, we can start by exploring your symptoms and health history together. Sometimes a SIBO breath test can offer clarity; other times, simpler steps may be more helpful. We’ll work out the most appropriate next step in a way that feels supportive and manageable.
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