This patient information is for patients who are doing a face-to-face breath test in the hospital to test for either fructose or lactose malabsorption. Please read it carefully before you arrive for your test. If you are not taking antibiotics, please complete your test within the next two weeks. Please pay special attention to the section titled 'How do I prepare for my breath test?' as we may not be able to perform the test if this is not followed.
If you are being tested for fructose malabsorption, please read 'What is fructose malabsorption?' below.
If you are being tested for lactose malabsorption, please read 'What is lactose intolerance?' below.
What is fructose malabsorption?
Fructose is a sugar which is found naturally in fruits. It is normally absorbed in the small bowel but if not fully absorbed it can lead to symptoms of cramping, bloating and diarrhoea. Symptoms vary depending on the person and the amount of fructose they have eaten. Fructose malabsorption can occur in people with no other underlying illnesses.
What is lactose intolerance?
Lactose is a sugar which is found in milk and dairy products. Some people are unable to digest lactose because they do not produce enough of an enzyme called lactase. Enzymes are substances which help to break down food. When the undigested lactose reaches the large bowel it can cause symptoms of cramping, bloating and diarrhoea.
Lactase deficiency occurs in approximately 1 in 20 of the UK population, but rates differ in different ethnic groups. Symptoms may vary depending upon the amount of lactose you eat. Lactose intolerance may occur in people with no other underlying illnesses.
What are the risks of having this test?
There are no risks involved with having a breath test. If you are lactose intolerant you may also experience your usual symptoms after the drink.
Are there any alternatives to this test?
There are no alternative tests to investigate these issues at UCLH. If you decide not to do this test, we will let your referrer know who will then discuss follow-up options with you.
How do I prepare for my breath test?
Medications:
- Do not take probiotics for 4 weeks before the test. Probiotic foods such as yogurt or fermented foods are not included and can be eaten.
- Do not take antibiotics for 4 weeks before the test. If you are on a short course, then please continue the medication and schedule the test 4 weeks after finishing. If you routinely take daily antibiotics, you do not need to stop them before doing the test.
- Do not take laxatives for 1 week before the test (if you only take them occasionally). If you routinely take laxatives daily, continue to take these and stop 24 hours before the test.
- Do not take medications that affect your gut motility for 24 hours before the test (e.g. metoclopramide, prucalopride, baclofen).
The day before your test:
- Please avoid high fibre foods e.g. beans, pulses, lentils, vegetables such as broccoli and cauliflower, and wholemeal products.
- Please do not have anything to eat or drink (except for water) for 13 hours before your test. This usually means fasting after your evening meal and skipping breakfast. Please do not chew gum.
- Please do not smoke for two hours before your test.
- Brush your teeth as normal in the morning but do not use mouthwashes which contain alcohol.
NB: If you are diabetic, please consider the fact that you will be drinking the equivalent of a medium or small sized carbohydrate meal (25g of fructose or lactose). If you are insulin dependent, you may need to alter your dose accordingly.
What will happen during the test?
The test is simple and will last up to 180 minutes. A clinician will complete the pre-test checklist with you and explain the test. Once you are happy to proceed, they will demonstrate how to take a breath sample by breathing through a mouthpiece. The mouthpiece is attached to a device which will measure specific gases in your breath.
Once you have completed the first breath sample, you will be asked to drink a sugary drink (fructose or lactose mixed with water). You will then be asked to wait in the waiting area until your next sample. You will complete 10 samples in total over 180 minutes.
You cannot eat or drink during the test. You cannot smoke or use alcohol hand gel. You must limit movement during the test.
You may wish to bring something to keep you occupied whilst you wait.
What will happen after the test?
The clinician will inform you of the result of the test once you have completed all the samples. A report will be written and sent to you, your referrer and your GP.
When you return to see your doctor (for example at the outpatient clinic at the Trust, at another hospital, or your GP surgery) the test results and any treatment will be discussed with you.
You will be able to go home and continue with your normal activities after the test.
Contact details
This page tells you about hydrogen / methane breath tests but if you have read the page and have any queries, please contact the GI Physiology Unit.
Address: GI Physiology Unit, Lower Ground Floor, EGA Wing University College Hospital, 25 Grafton Way London, WC1E 6DB
GI Physiology Administrator: 020 3447 9130 (Monday to Friday 09:00 to 17:00)
Switchboard: 020 3456 7890 / 08451 555 000, ext. 79130
Email: UCLH.
How to find us
The GI Physiology Unit is on the lower ground floor of the EGA wing.
Page last updated: 28 October 2025
Review due: 31 October 2027