Small intestinal bacterial overgrowth — SIBO — is a condition in which bacteria that normally live in the large intestine migrate into and proliferate in the small intestine. It is one of the most frequently missed causes of IBS, persistent bloating, and unexplained fatigue, and it is rarely tested for on the NHS. This guide explains what a SIBO breath test measures, how to access one in the UK, and what to do with your results.
What Is SIBO?
The small intestine is not sterile, but it should contain relatively few bacteria compared to the large intestine. When bacteria overgrow in the small intestine, they ferment carbohydrates before they can be properly absorbed, producing hydrogen and methane gases that cause bloating, distension, pain, altered bowel habits, and nutrient malabsorption.
SIBO is estimated to be present in 30–85% of people with IBS, depending on the diagnostic criteria used. It is also associated with hypothyroidism, coeliac disease, Crohn's disease, previous abdominal surgery, and proton pump inhibitor use. Many people with SIBO have been told they have IBS and managed symptomatically for years without the underlying cause being identified.
How Does a SIBO Breath Test Work?
A SIBO breath test measures the gases produced by bacterial fermentation in the small intestine. You drink a substrate — either lactulose or glucose — and then breathe into collection tubes at regular intervals over two to three hours. The breath samples are analysed for hydrogen and methane gas.
Hydrogen is produced by bacteria fermenting carbohydrates. Methane is produced by archaea and is associated with constipation-predominant SIBO. An early rise in hydrogen or methane — before the substrate reaches the large intestine — indicates bacterial overgrowth in the small intestine.
| Gas | Associated Symptoms | SIBO Type | |
|---|---|---|---|
| Hydrogen | Diarrhoea, loose stools, bloating | Hydrogen-dominant SIBO | |
| Methane | Constipation, slow transit | Methane-dominant SIBO (IMO) | |
| Both | Mixed bowel habits, bloating | Mixed SIBO |
Lactulose vs Glucose: Which Test Is Better?
Glucose is absorbed in the first part of the small intestine, so a glucose breath test only detects SIBO in the proximal small intestine. Lactulose is not absorbed and travels the full length of the small intestine, making it more sensitive for detecting SIBO further along the gut. However, lactulose also produces a natural rise in gases when it reaches the large intestine, which can be misinterpreted as a positive result.
Most functional medicine practitioners prefer lactulose breath testing with careful interpretation of the timing and pattern of gas rises. Elena uses lactulose-based SIBO testing as part of the Advanced Functional Health Assessment when SIBO is clinically suspected.
Where to Get a SIBO Breath Test in the UK
SIBO breath testing is not routinely available through the NHS. NHS gastroenterology departments do offer hydrogen breath tests for lactose and fructose intolerance, and some specialist centres offer SIBO testing, but waiting times can be long and access is inconsistent.
Functional medicine practitioners — Elena orders SIBO breath tests through accredited laboratories as part of a comprehensive assessment. This is the most clinically useful route because results are interpreted in the context of your full symptom picture and health history.
Private gastroenterology clinics — Some private gastroenterologists offer SIBO breath testing, typically as part of a broader IBS workup.
At-home breath test kits — Several companies offer postal SIBO breath test kits. These can be useful for initial screening but require careful interpretation; a positive result on an at-home kit does not automatically mean you have SIBO, and a negative result does not rule it out.
How to Prepare for a SIBO Breath Test
Preparation is critical for accurate results. Standard preparation includes a specific preparatory diet for 24–48 hours before the test, avoiding fermentable carbohydrates and fibre. No antibiotics, probiotics, or prokinetics for two to four weeks before testing. No eating, drinking (except water), exercising, or smoking on the morning of the test. Your practitioner will provide specific preparation instructions based on the laboratory protocol being used.
Interpreting SIBO Breath Test Results
Interpreting SIBO breath test results requires clinical expertise. The North American Consensus guidelines define a positive hydrogen result as a rise of 20 ppm or more above baseline within 90 minutes of substrate ingestion. Methane positivity is defined as 10 ppm or more at any point during the test. However, these thresholds are not absolute — the pattern of gas rise, the substrate used, the preparation protocol, and the clinical context all affect interpretation.
Treatment for SIBO
SIBO treatment depends on the gas type, severity, and underlying cause. Antibiotics — Rifaximin is the most commonly used antibiotic for hydrogen-dominant SIBO. Methane-dominant SIBO typically requires a combination of rifaximin and neomycin. Herbal antimicrobials — Herbal protocols using compounds such as berberine, oregano oil, and allicin have evidence for efficacy in hydrogen-dominant SIBO. Elemental diet — A two-week elemental diet has high efficacy for SIBO but is challenging to follow.
Without addressing the underlying cause of SIBO — whether that is low stomach acid, impaired motility, structural issues, or immune dysfunction — relapse rates are high. This is why SIBO treatment in functional medicine always includes root-cause investigation. For people with IBS and digestive issues, SIBO is one of the first conditions Elena investigates.
Frequently Asked Questions
Can I get a SIBO breath test on the NHS?
Standard SIBO breath testing is not routinely available on the NHS. Some NHS gastroenterology departments offer hydrogen breath tests for carbohydrate malabsorption, but these are not the same as a full SIBO assessment. Private testing through a functional medicine practitioner is the most reliable route.
How long does a SIBO breath test take?
The test itself takes two to three hours. You will need to breathe into collection tubes every 20 minutes after drinking the substrate. At-home kits allow you to do this at home and post the samples to the laboratory.
Can SIBO come back after treatment?
Yes. SIBO relapse is common, particularly if the underlying cause is not addressed. Recurrence rates of 40–50% within nine months have been reported. A comprehensive treatment plan that addresses root causes — not just bacterial eradication — is essential for long-term resolution.
Is SIBO the same as IBS?
No, but they frequently co-exist. SIBO is a specific condition with a measurable bacterial overgrowth. IBS is a symptom-based diagnosis. Many people diagnosed with IBS have undetected SIBO as the underlying cause of their symptoms.
When to Seek Practitioner Support
If you have persistent bloating, altered bowel habits, abdominal pain, or fatigue that has not responded to dietary changes, it is worth investigating SIBO with a qualified practitioner. If you have blood in your stool, unexplained weight loss, or a family history of bowel cancer, see your GP first to rule out more serious conditions before pursuing private testing.
Take the Next Step
Book a free discovery call with Elena to discuss whether SIBO testing is appropriate for your symptoms. She can order the correct breath test, interpret the results in full clinical context, and design a personalised gut reset programme to address both the overgrowth and its root causes.