SIBO is increasingly recognised as a major cause of IBS.  In fact studies have shown that the incidence of SIBO in IBS sufferers could be as high as 84%. But the symptoms are quite similar to other gut disorders. So a SIBO test can be a very reliable way to start to address the condition.

SIBO is caused by an overgrowth of commensal bacteria, which are regular inhabitants of the gut.

Tests for SIBO examine the gases that the small intestine produces to determine if overgrowth is a problem.

In the UK there are 2 key SIBO tests. Both are hydrogen breath tests, using either lactulose or glucose. Choosing between the two and understanding the methodology may not be obvious to most people.

Read on to learn more about SIBO testing, and why it could be important for you:

At IBS Clinics we recommend special diets to aid the eradication of SIBO. The key diets used reduce the amount of fermentable carbohydrates consumed which provides symptomatic relief within a few days. The diets are tailored to the individual based on your personal circumstances and also the type and severity of SIBO you have. Let's discuss whether this diet could be helpful for you.

Primary SIBO symptoms

  • Bad bloating
  • Excessive gas
  • Diarrhoea 
  • Nausea
  • Abdominal pain
  • Fatigue
  • Food intolerances
  • Unwanted weight loss

and even:

  • Vitamin and mineral deficiencies that can lead to malnutrition

And it’s really important to look out for:

  • constipation

A Quick Overview of SIBO

First let’s recap on what SIBO is:

Small Intestine Bacterial Overgrowth (SIBO) is where there is an increase in the number and/or an alteration in the type of bacteria in the upper gastrointestinal tract.


In your smaller intestine, these bacteria ferment any carbohydrates that we have eaten and as a by-product, gas is released. As well as aiding in the digestion of our food, as part of the deal they absorb some nutrients for themselves. This can result in these SIBO symptoms (see left panel).

In fact there are 2 main types of SIBO and constipation can point to one SIBO rather than another. In the main type of SIBO the fermentation in the small intestine releases hydrogen gas, so people with SIBO have elevated levels of hydrogen in their GI tract.

But SIBO can also cause an increase in methane levels.  Hydrogen feeds single-cell organisms in your gut called archaea which produce methane. As methane is made from hydrogen like this, so you can have a false negative test if hydrogen is the only gas tested. (This sometimes occurs in NHS tests).  But SIBO tests that include both gases sometimes record high methane and no hydrogen gas.

So how do you test for SIBO?

A noninvasive test is always preferred if possible and breath testing for SIBO is frequently the goto method of diagnosis. However, it does involve a restrictive diet beforehand.


Most importantly the test can be taken at home, although we highly recommend having a specialist review the results for you.

How does it work?

The presence of methane and hydrogen is a telltale sign of SIBO so the test detects levels of these gasses in the breath. A solution of a sugar such as lactulose or glucose is consumed and the gasses are tested for at specific time periods. 


If hydrogen is detected in the breath at raised concentrations at around 90 minutes after consumption of the sugar solution, this is an indication that bacteria in the small intestine has been detected. Also levels of methane above 10 parts per million goes some way to proving SIBO-Methane


The sugar solution that was  ingested normally takes a couple of hours to find its way to the large intestine and to interact with the indigenous bacteria that it finds there.

methane and hydrogen atoms in red and blue

Which SIBO breath test is best?

Both lactulose and glucose tests are common, but which is the best? 

Lactulose can give results which are false positives, whereas using glucose for a SIBO breath test is usually accurate. However, lactulose identifies SIBO through the entirety of the small intestine,  in comparison to glucose. The most problematic aspect of either type of breath test for SIBO is the difficulty in distinguishing between gasses from the small and large intestines. Which is clearly a problem.


Talk to us or before selecting a test because breath testing for SIBO is more effective when a skilled practitioner interprets the results. The IBS Clinics team will take all symptoms into consideration in conjunction with testing data.

Getting ready for the breath test

The at-home SIBO test requires a restrictive diet on the day before the test and an overnight fast. The restrictive diet starts at lunch time on the day before you are going to take the test. We will of course provide detailed guidance on the diet, but broadly the following foods are ok:

  • Chicken or turkey
  • Fish
  • Eggs (poached or boiled only)
  • Hard cheeses
  • Clear chicken or meat broth 
  • White rice or white bread 

Overnight fasting starts after dinner on the day before the test.

Other SIBO Tests

Jejunal Aspiration Test

Medication Trial

This is an invasive test that is largely considered to have more accuracy than a simple breath test when searching for a SIBO diagnosis. This test is undertaken during a procedure known as an upper endoscopy. So a trip to a GP or a specialist doctor is going to be a requirement with this test.

How does it work?

A sample is cultured that has been obtained from the small intestine. The sample which is from the intestine’s middle section has its bacterial presence evaluated.


The accuracy of this test is also limited in its scope to identify SIBO cases. Cross contamination from other parts of the small intestine is always a risk. Jejunal aspiration testing is not generally used. It is costly, time-consuming, and while generally considered safe, still carries more risks than a breath test.

Occasionally gastroenterologists may attempt to determine the presence of SIBO via experimental treatment. Rapid relief of symptoms is suggestive of the presence of SIBO.

The antibiotic Xifaxan (or Rifaximin) is commonly diagnosed to treat SIBO. This antibiotic works in a different manner to traditional ones as it works locally in the gut rather than being digested and distributed in the bloodstream.

The most commonly used medication for SIBO is the antibiotic Xifaxan (or Rifaximin). This antibiotic is different from most others as it is not absorbed into the body through your stomach. Instead, it acts locally on any bacteria that might be in your small intestine.

The issue with this approach is that if your have very high levels of bacteria overgrown in your small intestine a single course of antibiotics are unlikely to be effective.  SIBO testing pre and post antibiotics is sensible as a guide to treatment.

Also, as with many antibiotics Xifaxan can quite often have side-effects, including:

  • A Strong Desire To Have Bowel Movement
  • Dizziness
  • Fluid Retention In The Legs, Feet, Arms Or Hands
  • Gas
  • Headache
  • Ineffective Painful Straining Of Stool Or Urine
  • Low Energy
  • Nausea
  • Stomach Cramps

What should you do next?

If you are considering medication or Jejunal Aspiration then talk to your GP as a first step.

If you have decided that you want to have a SIBO test, and think that a breath test is the way that you will go then please talk to us before you order a test online. A quick call will mean that we can help you choose the right test.