Hydrogen breath test (Southmead Hospital)

This page outlines what to expect both during and after the test. You will need to do some test preparation, so please read everything carefully.

Hydrogen breath tests 

A hydrogen breath test is a safe, non-invasive test used to investigate certain intestinal disorders. Your consultant will have referred you to assess whether you have one or both of the following:

• Small bowel bacterial overgrowth: This is a condition where there are abnormal numbers of bacteria in the small intestine.

  or

• An intolerance to a specific sugar: An intolerance is when the body is unable to sufficiently break down a specific food (e.g. lactose which is the sugar found in dairy products).

Please be aware that if you have been referred for more than one breath test, these cannot be performed on the same day.

Upon arrival 

You will be greeted and taken into a private room by a member of the GI physiology team who will be performing the test. They will ask you some questions about the symptoms you have been having and take a brief history. They will also take this opportunity to answer any questions you have or any concerns regarding the test.

How shall I prepare for the test?

• You need to fast for at least 14 hours before the test and not drink anything apart from water (particularly avoid milk and fruit juice the day before the test). If you are diabetic, please see the frequently asked questions below.
• Avoid high fibre foods the day before the test including: fruit, vegetables, wholegrains.
• Try to base meals around white bread, plain white rice/ pasta, potatoes (skin off), chicken or fish. There is a suggested meal plan below.
• Your last meal on the day before the test should not be too big and should ideally not consist of any roughage (e.g. beans, cabbage or leeks).
• You must not smoke/vape or chew gum 12 hours before the start of the test.
• Avoid laxatives for 3 days before the test (especially lactulose).
• Medicines (apart from vitamins, laxatives and antibiotics) can be taken with plain water on the day of the examination.
• If you use dentures, do not use an adhesive on the day of test.
• Brush your teeth on the day of the test.

The following will make the test results difficult to interpret:

• Antibiotics in the last 4 weeks.
• Colonoscopy in last 4 weeks.
• Irrigoscopy in last 4 weeks.
• Other bowel cleansing procedures in last 4 weeks.
• Ileostomy (except for diagnosis of bacterial overgrowth).

Please contact the department if any of the above is unclear or you have had any in the last month (see below).

What does the test involve?

• You will be asked to give an initial breath sample into a hand-held machine. This is done by holding your breath for 15 seconds and then blowing gently into a mouthpiece at a steady rate.
• A cup of sugar dissolved in water will be then given to you to drink.
• Additional breath samples will be taken every 20 minutes for 2-3 hours, while the test solution travels through the gastrointestinal tract.
• During the test you should not eat, chew gum, smoke, sleep, or exercise.
• You can continue normal activities once the test is completed.

Repeat testing 

If your initial breath sample is not sufficiently low enough (e.g. due to poor test preparation), we may have to re-book the test for another day. If this happens, we will ask you to come back with the following instructions:

• Fast for at least 16 hours before the test.
• Don’t eat any roughage (fibrous foods like beans, cabbage, and leeks) before the test.
• If lactose intolerance is a possibility the last meal consumed prior to the test should not contain milk or dairy products. If fructose intolerance is suspected the last meal should not contain fruit.
• On the morning of the test drink a glass of warm water (200-300ml).