How Hydrogen Breath Tests Work

This test may help in diagnosing SIBO and lactose intolerance

A hydrogen breath test is used to measure the amount of hydrogen gas in a person’s breath. It can be used as part of diagnosing some digestive conditions, especially small intestinal bacterial overgrowth (SIBO), carbohydrate malabsorption (including lactose intolerance), and if food is moving too slowly through the small intestine.

This type of test was first described in 1975, but it has not always been widely used. People may be given the option of taking this test if they have symptoms of abdominal pain, bloating, gas, constipation, or diarrhea that are going on for a long time (chronic).

This article will discuss why a hydrogen breath test might be used, how the test is done, and what the results might mean.

A doctor discussing instructions with a patient.

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How Hydrogen Breath Tests Work

Sugars in food are broken down during digestion. If you do not digest some sugars well, the bacteria in the small bowel will give off hydrogen. The amount of hydrogen in the breath is normally low. When it is too high, it is one sign that certain types of food are not being digested well and absorbed by the body.

Types of Hydrogen Breath Tests

There are a few different types of hydrogen breath tests. You will be given a solution to drink that contains a specific type of sugar.

Lactose Breath Test

Lactose is the sugar that’s found in milk. Not being able to digest milk sugar is a common condition called lactose intolerance. In this test, you will drink a solution that contains lactose.

Fructose Breath Test

Fructose is a type of sugar that is found in honey, vegetables, fruits, and grains. Not being able to digest fructose fully is common. In this form of the test, patients are given a drink that contains fructose. 

SIBO Breath Test

The overgrowth of bacteria in the small intestine can result in the production of too much hydrogen. For this test, you will drink a solution containing glucose.


The hydrogen breath test is noninvasive and inexpensive. However, it will require some preparation. The healthcare provider will give you specific steps to follow and advise you on which medications to avoid and what dietary changes you should make.

In the month (four weeks) before having a hydrogen breath test, you should stop taking: 

In the one or two weeks before the test, you should avoid taking:

In the two days before the test, you will be asked to eat a diet that’s low in carbohydrates and dairy products. This includes avoiding foods with wheat, beans, milk products, foods with high fructose corn syrup, natural and artificial sweeteners, and fruits and vegetables containing high fructose levels.

It isn’t easy or simple to know exactly which foods to avoid and which are allowed, so your healthcare provider should give you a list. If a list isn’t provided, you should ask for one to ensure you’re following the right diet.

On the day of the test, you will also be asked to:

  • Avoid exercising or sleeping in the hour before the test.
  • Avoid mouthwash, mints, or gum (because they may contain sugars).
  • Fast for eight hours before the test.
  • Stop smoking or vaping (or being around secondhand smoke).


The procedure is fairly straightforward but will take two to three hours. The first step is to get a baseline breath sample. You will breathe into a bag to collect the sample.

Next, you will drink the solution that contains a type of sugar. You will then be asked to breathe into a plastic bag every 15 or 20 minutes. For children, this interval might be every 30 minutes. 

Each sample of breath taken will be measured for its hydrogen content. The measurement is done with a device called a gas chromatograph.

Interpreting Results

The level of hydrogen in the breath is measured in parts per million (ppm). The test samples will be compared against the initial sample taken before the test began.

When the amount of hydrogen in a test sample is higher than the first sample, it may indicate a problem with absorption in the small intestine. A rise of 10 parts per million (ppm) or more might mean a glucose or fructose intolerance. A 20 ppm or more rise might mean that lactose intolerance or SIBO is present.

The healthcare provider who ordered the test will help in explaining what the results might mean. If the results indicate a condition that needs treatment, you may be given a referral to a specialist.

There are, however, some problems that can occur with these test results, which can result in a false positive or a false negative.

A false positive is when the test shows a high level of hydrogen, but it isn’t caused by a condition that needs to be treated. A false positive could be caused by the balance of bacteria in the mouth, from food moving too quickly through the small intestine, or from eating carbohydrates or dairy products before the procedure.

A false negative is also possible. This is when there is actually a condition present, but the test doesn't indicate the high level of hydrogen. This happens in about 10% of tests.

One reason for a false negative is that the flora in the microbiome doesn’t produce enough hydrogen to be picked up by the test. Another reason for false negatives could be slow transit time. This means that it’s taking longer than normal for food to move through the small bowel. 

It’s worth discussing the potential for false-positive and false-negative results with a healthcare provider. In some cases, more tests may be needed. 


When there is a problem with digesting certain types of sugars, excess hydrogen might be a byproduct. The hydrogen breath test is used to measure excess hydrogen levels in the breath and to confirm or rule out a diagnosis of SIBO, lactose intolerance, or other conditions.

A Word From Verywell

The hydrogen breath test is helpful in finding the cause of digestive symptoms. Lactose intolerance has been recognized for a long time, but not being able to digest sugars such as fructose is not as well known. The hydrogen breath test can detect this.

SIBO is also increasingly understood and diagnosed, especially in the setting of other digestive conditions, such as irritable bowel syndrome (IBS). The hydrogen breath test may not be the only tool to determine the cause of symptoms, but it is relatively straightforward and is increasingly available. 

Frequently Asked Questions

  • Are hydrogen breath tests accurate?

    Yes, the hydrogen breath test is considered to be pretty accurate. However, it’s important that all the instructions about diet and medications are followed in the weeks and days leading up to the test. Not following them might make the test inaccurate. 

  • Should you fast before a hydrogen breath test?

    Yes, it will be necessary to fast before this test. It’s important to get results from the drink that’s given and not other foods that are eaten or drunk.

  • How long do hydrogen breath tests take to complete?

    The time leading up to the test can be several weeks while diet and medications are adjusted. The test itself should take two to three hours to complete. 

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Robles L, Priefer R. Lactose intolerance: what your breath can tell you. Diagnostics (Basel). 2020;10:412. doi:10.3390/diagnostics10060412.

  2. Rezaie A, Buresi M, Lembo A, et al. Hydrogen and methane-based breath testing in gastrointestinal disorders: The North American consensus. Am J Gastroenterol. 2017;112:775-784. doi:10.1038/ajg.2017.46. 

  3. Hammer HF, Fox MR, Keller J, et al. European guideline on indications, performance, and clinical impact of hydrogen and methane breath tests in adult and pediatric patients: European Association for Gastroenterology, Endoscopy and Nutrition, European Society of Neurogastroenterology and Motility, and European Society for Paediatric Gastroenterology Hepatology and Nutrition consensus. United European Gastroenterol J. 2022;10(1):15-40. doi:10.1002/ueg2.12133

  4. Takakura W, Pimentel M. Small intestinal bacterial overgrowth and irritable bowel syndrome - an update. Front Psychiatry. 2020;11:664. doi:10.3389/fpsyt.2020.00664. 

Additional Reading

By Amber J. Tresca
Amber J. Tresca is a freelance writer and speaker who covers digestive conditions, including IBD. She was diagnosed with ulcerative colitis at age 16.