Hydrogen Breath Test

  • Reference Number: HEY-092/2018
  • Departments: GI Physiology


This leaflet has been produced to give you general information about your forthcoming hydrogen breath test.  Most of your questions should have been answered by this leaflet. It is not intended to replace the discussion between you and your doctor.  If after reading it you have any concerns or require further explanation, please discuss this with a member of the healthcare team.

What is a Hydrogen Breath Test?

A hydrogen breath test is a non-invasive and safe test used to investigate intestinal disorders.  You will be given a test solution to drink after you have fasted (not eaten) overnight.  The concentration of hydrogen (measured in parts per million) in your breath is then measured using a hydrogen breath test machine.  If your breath contains a large amount of hydrogen (more than 20 parts per million above your baseline) it will be a positive test.  The baseline is the amount of hydrogen that is present in your breath before drinking the test solution.

Why do I need a Hydrogen Breath Test?

A hydrogen breath test provides information about the digestion of certain sugars or carbohydrates.  This will help determine if you are intolerant to, or cannot digest, certain sugars. One example is lactose (milk sugar) intolerance, a disorder in which people develop symptoms after eating foods such as dairy products or which contain lactose as an additive.

The test is also used for detecting abnormal growth of bacteria within the small bowel. It is normal for bacteria to live in your large intestine as they aid digestion. However, the small intestine does not usually contain bacteria. Sometimes bacteria can grow in the small intestine and interfere with the digestion of food; this is a condition known as Small Intestinal Bacterial Overgrowth (SIBO).  We usually use lactulose (a non-digestible sugar) for this test as it travels through the whole bowel. We also use glucose if we are particularly interested in the upper part of the small bowel.  Bacterial overgrowth can cause a variety of symptoms including nausea, diarrhoea, bloating, gas, and abdominal cramps.

Can there be any complications or risks?

The test substance is a sugar solution i.e. lactose, glucose or lactulose. The test in some cases (although rare) has been known to cause a small amount of diarrhoea.

How do I prepare for a hydrogen breath test

Please read this information leaflet. Share the information it contains with your partner and family (if you wish) so that they can be of help and support.

Four (4) weeks before your test:

You should not take any antibiotics. If you have to take antibiotics please contact us to re-book the test.

One (1) week before your test:

You should not undergo any test that requires cleansing of the bowel, such as colonoscopy or barium enema in the week before your hydrogen breath test.  Do not take any laxatives, stool softeners, stool bulking agents or any medication used to treat constipation.

The day before your test:                       

Please follow the diet sheet for your meals the day before the test.  If you are diabetic please telephone the department (01482) 622155 for advice before your study.

Diet Sheet – Menu for the day prior to hydrogen breath testing:

Drink fluids as normal (non-flavoured black coffee or tea and juice only).  Fizzy drinks such as cola or lemonade are not permitted.


Rice Krispies or any other shop brand of this type of cereal


Eggs (scrambled/poached/fried) and bacon

Toasted white bread (white bread only not brown)


Chicken or fish (no batter / breadcrumbs)

Boiled or freshly mashed potatoes or chips (without skins)

Pudding: Small portion of ice-cream, sugar free jelly, sugar free custard, or natural plain yoghurt

Evening meal:

Cheese omelette (no vegetables)


Eggs (scrambled/poached/fried)

Toasted white bread (white bread only not brown)


Ham or cheese sandwich (white bread only not brown)


Rich Tea biscuits

(No fruit, vegetables or wholegrain products)


It is very important that you do not eat or drink [except water] from 6pm the night before the test.

The day of your test:

You should not eat or drink anything in the morning, a little water is permitted on the morning of the test if you need to take any medication.

On the morning of your test, please brush your teeth but be careful not to swallow the toothpaste.


Do not sleep or exercise while the test is being done.

What will happen?

Please attend the department of GI physiology, which is situated adjacent to Ward 14 (1st floor).  This is located via Entrance 2, Main reception, Castle Hill Hospital.

A member of staff from the GI Physiology Department will explain the hydrogen breath test to you.  This is to ensure that you understand the test and its implications.  The test is very easily performed.  It requires you to breathe down a mouthpiece into a hand held machine that measures hydrogen.  The breath samples are collected whilst you are blowing at your normal rate, therefore this should not make you short of breath.

Providing you have fasted correctly, the first reading should be very low.  If this reading is excessively high, we may have to abort the test and ask you to return in a few hours, or re-book the test for another day.  You will drink the test solution (either lactose, glucose or lactulose).  You will then breathe into the machine at regular intervals for up to 4 hours, so you may want to bring some reading material along with you.

After the test has finished you may go home or to work.

What happens afterwards?

The consultant in charge of your care will be informed of your results and will decide on your treatment plan.  It is reasonable to expect to hear from your consultant (with an outpatient follow up appointment) regarding your results approximately 4 weeks after having your test.

Should you require further advice on the issues contained in this leaflet, please do not hesitate to contact the GI Physiology Department on (01482) 622155

General Advice and Consent

Most of your questions should have been answered by this leaflet, but remember that this is only a starting point for discussion with the healthcare team.

Consent to treatment

Before any doctor, nurse or therapist examines or treats you, they must seek your consent or permission. In order to make a decision, you need to have information from health professionals about the treatment or investigation which is being offered to you. You should always ask them more questions if you do not understand or if you want more information.

The information you receive should be about your condition, the alternatives available to you, and whether it carries risks as well as the benefits. What is important is that your consent is genuine or valid. That means:

  • you must be able to give your consent
  • you must be given enough information to enable you to make a decision
  • you must be acting under your own free will and not under the strong influence of another person

Information about you

We collect and use your information to provide you with care and treatment. As part of your care, information about you will be shared between members of a healthcare team, some of whom you may not meet. Your information may also be used to help train staff, to check the quality of our care, to manage and plan the health service, and to help with research. Wherever possible we use anonymous data.

We may pass on relevant information to other health organisations that provide you with care. All information is treated as strictly confidential and is not given to anyone who does not need it. If you have any concerns please ask your doctor, or the person caring for you.

Under the General Data Protection Regulation and the Data Protection Act 2018 we are responsible for maintaining the confidentiality of any information we hold about you. For further information visit the following page: Confidential Information about You.

If you or your carer needs information about your health and wellbeing and about your care and treatment in a different format, such as large print, braille or audio, due to disability, impairment or sensory loss, please advise a member of staff and this can be arranged.