How to test for SIBO in the UK

If you’ve been dealing with bloating, constipation, diarrhoea, or abdominal pain and have heard of SIBO (Small Intestinal Bacterial Overgrowth), you might be wondering how to find out if you have it.

Testing for SIBO isn’t always straightforward, but there are options available to you in the UK.  This blog explains how SIBO breath testing works, what symptoms might warrant testing, and what to expect from the results.

What is SIBO?

SIBO stands for Small Intestinal Bacterial Overgrowth. It happens when there is an abnormal increase in the number or type of bacteria in your small intestine.

These bacteria ferment carbohydrates and produce gas as a by-product, which can lead to symptoms like:

• Bloating and distension

• Diarrhoea or constipation

• Excessive gas (burping or farting)

• Abdominal pain and sensitivity

• Nutrient deficiencies (in long-standing cases)

When should you test for SIBO?

Testing for SIBO may be helpful if you:

• Have persistent IBS symptoms that haven’t responded to dietary changes (like the Low FODMAP diet) or the traditional dietary advice (e.g. reduce coffee, alcohol, eat regularly, eat fruit and vegetables)
• Have ongoing bloating, gas and a noisy digestion, especially 1-3 hours after eating
• Have a history of food poisoning or heavy antibiotic use
• Notice your digestion is better when you take antibiotics

Ideally you’d work with a nutritionist before testing to work out if you need a test.

You could be wasting your time and money on testing for SIBO but actually need another more basic issue addressed first. For example, you could get resolution of your symptoms through identifying a food trigger, and eating a wider variety of foods.

Before you test for SIBO

Before you get a test I recommend you:

• address your diet – aim to eat a good range of fruits, vegetables, protein and wholegrains
• reduce gut triggers such as caffeine, alcohol, smoking
• visit your doctor to rule out other conditions – some symptoms can overlap with other conditions like coeliac disease, IBD, or bile acid diarrhoea.

What are the options for SIBO testing?

The SIBO breath test works by measuring the level of hydrogen or methane gas in your breath. If you’re wondering why it would be in your breath, it’s a good question!

When bacteria in your small intestine ferment certain carbohydrates (like those found in FODMAPs), they produce gases. These gases don’t just stay in your gut — some of them come out as farts, some get absorbed through the wall of the small intestine into your bloodstream.

From there, the gases travel through your blood to your lungs. When you breathe out, small amounts of those gases are released in your breath. That’s how we can measure them using a breath test.

A hospital may occasionally carry out a biopsy of your small intestine, but this is rarely done due to the cost, and it’s also an invasive procedure. The best way to diagnose SIBO is a lactulose or glucose breath test. These tests measure hydrogen and methane gases in your breath after you drink a sugar solution.

There are three main ways to test for SIBO: lactulose, glucose and fructose

Lactulose breath test

Lactulose is a synthetic disaccharide that is doesn’t get broken up or absorbed by our cells. This means it can detect SIBO throughout the small intestine.

Glucose breath test

Glucose might be a better food for the microbes, but it may miss overgrowth towards the end of your small intestine because it get absorbed into the cells.

Fructose breath test

Used less frequently, but there is potential for it to pick up some SIBO microbes.

How does the breath test work?

The process of taking a SIBO test is not without challenges. I warn my clients it’s a bit of a hassle.

Prep diet

For the entire day before the test (and two days if you’re very constipated) you should eat a very low-fermentation prep diet. This includes only plain white rice, chicken/fish, eggs. You can have tea and coffee, but no milk. You can use a small amount of oil to cook your food.

Fast for 12 hours

The next stage is to fast for 12 hours before the test (you are allowed to drink water). This might look like finishing your dinner at 7pm, and then not eating until 7am, when you can take the test next morning.

Take the SIBO test

During the test you’ll take a baseline breath reading, then you’ll drink either a lactulose or glucose solution. Breath samples are collected by breathing into little every 15–20 minutes for 2–3 hours.

Then you’ll either get results from the hospital, or you’ll send off the test kit to a lab to be analysed.

Interpreting breath test results

When you’re looking at your SIBO results you should see a graph showing the results of your gas levels in ppm (parts per million). You should also be able to read the numbers in a table.

The following readings indicate a positive result.

• For hydrogen – a rise of 10ppm (parts per million) within the first 60 minutes (EU guidelines) or a rise of 20ppm within 90 mins (US guidelines).
• Methane levels ≥10ppm at any time could point to Intestinal Methanogen Overgrowth (IMO).
• A flat-line result may suggest hydrogen sulphide gas overproduction, but we don’t have a way to test for this gas in the UK.

Stool tests for SIBO

Although a stool test isn’t diagnostic for SIBO, it will show the levels of methanogens in your large intestine. The main methane producing microbe is Methaninobrevibacter smithii.

Specific bacteria which produce a lot of methane are:

• Bacteroides thetaiotaomicron,
• Clostridium perfringens and Clostridium histolyticum

Are breath tests reliable?

Breath tests are non-invasive and widely used, but like any test, they’re not perfect.

They can give you incorrect results, such as 

• False positives (especially if bowel transit is very fast)
• False negatives (if testing too early or with the wrong substrate)
• Potential for differences in test interpretation between labs

We don’t know exactly how long your small intestine gut transit will be, and the rise in gas could be when the lactuluse hits your large intestine, where you want to have a lot of microbes. This is why some practitioners use a cut off of 60 minutes to see the result. 

That’s why it’s important to combine test results with symptoms and clinical judgement.

What about stool tests for SIBO?

Stool tests don’t diagnose SIBO directly, but they may:

• Show the presence of methanogens (like Methanobrevibacter smithii)
• Identify other imbalances such as dysbiosis or inflammation
• Provide clues if you have overlapping large bowel issues

Summary: What to expect with SIBO testing

Out of the SIBO breath tests my clients do around 80% of them are positive, but this is because I don’t recommend these tests unless I think they are necessary. So it’s an already self selected group. 

But we do know that SIBO can overlap with IBS symptoms, and if you can identify it as a cause of your digestive issues you can treat it. 

Breath testing is the most accepted non-invasive test for SIBO. It can be a hassle to do, and feel fiddly to manage at home on your own, but it can give accurate results if you can’t get access to a test via your doctor. 

Remember, your results should always be interpreted in the context of symptoms and not on their own. 

Final thoughts

If you suspect SIBO and want to find out for use, a breath test may be a useful next step — especially if you’re working with a qualified practitioner who can interpret the results and guide you through treatment. 

If you’d like me to help you with your digestion we can set up a free call to discuss what you’re looking for. I work with clients over 3 months in the Gut Reset and specialise in SIBO and IBS. 

PS – I give back any practitioner commission on SIBO tests to my clients so it is cheaper for you.