Test / Diagnostic Procedure: Hydrogen Breath Test

Breath tests are commonly used to investigate bowel symptoms such as diarrhoea, bloating or abdominal discomfort. The results of the test can help your gastroenterologist identify the cause of your symptoms so they can recommend the most appropriate and effective treatment for you.

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What is a hydrogen breath test?

A hydrogen breath test is a straightforward and non-invasive test that measures the levels of hydrogen and methane gases in your breath when you exhale. Under normal circumstances gases such as hydrogen and methane are produced by bacteria which live in the large bowel (colon) when they break down food. Gases are able to pass out of your gut into the bloodstream and circulate to the lungs where they are exhaled in your breath. The amount of hydrogen and methane in a sample of your breath can be measured using a special monitor.  We can use the change in the amounts of these gases over time (after a sugary drink) to test for bacterial overgrowth and malabsorption of some sugars.

What is Small Intestinal Bacterial Overgrowth (SIBO)?

Small intestinal bacterial overgrowth (SIBO) occurs when bacteria which are normally present in the colon have moved into the small bowel. This is where glucose is usually absorbed, but if bacteria have increased in that area they will break down the glucose drink we give to you and produce hydrogen.

Bacterial overgrowth usually occurs because of abnormal movement of the gut. It is most common in patients with illnesses which directly or indirectly affect the gut and less common in patients with no underlying illness or Irritable Bowel Syndrome.

What is Carbohydrate Malabsorption?

Carbohydrates (sugars) such as lactose and fructose are usually absorbed in the small bowel. If some of the sugar is not absorbed and reaches the large bowel, this is referred to as malabsorption. If a sugar reaches the large bowel the gut bacteria which live there break it down and produce gases such as methane and hydrogen, which we can measure using a breath test. Malabsorption of lactose or fructose can lead to symptoms such as cramping, bloating and diarrhoea and is this is called lactose or fructose intolerance.

Lactose is a sugar which is found in milk and dairy products and fructose is a sugar found commonly in fruits. Symptoms of intolerance can vary depending on the person and the amount of sugar they have eaten. An intolerance is not an allergy and for most people treatment involves being aware of the problem and lowering the dose of these sugars rather than complete avoidance.

Preparing for a hydrogen breath test

Some preparation is required ahead of taking a hydrogen breath test to ensure the results are as accurate as possible. You will be given full details by your specialist but all patients must:

  • Stop taking any antibiotics and probiotics one month before the test
  • Eat a low-fibre diet the day before the test.
  • Fast (consume no food or drink, except water) for at least 12 hours before the test

What does a hydrogen breath test involve?

The test is simple and non-invasive but is time-consuming so we recommend you bring something with you to read. You can access our Wi-Fi network in our waiting areas.

The first step of the test is to provide a fasting sample by holding your breath and then breathing out into a mouth piece attached to a specialist device that measures the gases in your breath sample. This records the base levels of hydrogen and methane in your breath so that the results of further samples from the test can be compared.

Once the fasting sample has been collected you will be given a sugar solution to drink (glucose, lactose or fructose). You’ll then give multiple breath samples at approximately 15 minute intervals, over the course of 90 to 180 minutes (depending upon which test you are undertaking). You may be asked to record any symptoms over this time.

Recovering from a hydrogen breath test

There are no risks involved in having a breath test. If you are intolerant of the sugar which you have been given, the drink may reproduce your typical symptoms but no harm can come from the test. You can go home as soon as the test has been completed, eat and drink normally straight away.


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