Hydrogen Breath Test

The hydrogen breath test is a simple and noninvasive way to diagnose common gastrointestinal conditions, including lactose intolerance and SIBO (small intestine bacterial overgrowth). It does take some advance preparation to make sure results are accurate. The breath test evaluates your digestion of particular sugars by measuring the gas you exhale, usually hydrogen and methane.


What is the hydrogen breath test?

The hydrogen breath test is a simple medical test that measures hydrogen gas levels (HO2) in the breath you exhale. It’s used to help diagnose common digestive problems, including SIBO, IBS and lactose intolerance. Different versions of the test measure your digestion of different sugars. The results help healthcare providers confirm or rule out certain specific causes of your digestive problems.


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What does the hydrogen breath test diagnose?

Many different digestive conditions will cause abnormal hydrogen levels in your gut, which can then be measured in your breath. But when your healthcare provider suggests the hydrogen breath test, it’s usually to confirm or rule out one of two conditions:

  • Intolerance to a specific sugar, such as lactose, fructose, sucrose or sorbitol — also called carbohydrate malabsorption. Lactose intolerance is the most common.
  • Small intestinal bacterial overgrowth (SIBO). This is when helpful bacteria from your colon have moved into your small intestine, where they don’t belong.

In the process of diagnosing irritable bowel syndrome (IBS), both of these conditions must be addressed as possible contributing factors.

Test Details

How does the hydrogen breath test work?

If you’ve ever experienced gas in your belly from indigestion — the bloating and swelling like a balloon, the painful movement of the gas through your digestive system and the uncomfortable process of farting it out — that gas was likely at least part hydrogen. If there’s hydrogen in your body, only one thing causes it: anaerobic gut bacteria digesting sugars and carbohydrates in your gut. This process is called fermentation.

These bacteria are supposed to live in your colon, where they play an important role in digestion. But sugars and carbohydrates aren’t supposed to be a major part of their diet. In a healthy digestive system, the sugars and carbohydrates that you eat are predigested before they reach the bacteria in your colon. If they haven’t been fully digested by then, the anaerobic bacteria will digest them, converting them into gas.

The gas in your colon is absorbed into your blood and carried into your lungs, where it’s expelled through your breath. That’s how your breath can show what’s going on in your gut. The hydrogen breath test measures hydrogen levels in your breath after consuming a particular sugar. The amount of hydrogen, and how fast it’s produced, gives your healthcare provider information about how your digestion is working.


Do breath tests measure other gases?

  • Methane: Because everyone’s assortment of gut bacteria is a little different, some people have more methane in their breath than hydrogen. For this reason, most clinics today measure for both gasses at the same time. Both are measured in the same way. However, the methane test is more recent and may not be as accurate as the hydrogen breath test.
  • Carbon dioxide: A different type of breath test measures carbon dioxide (CO2) levels in your breath. The H. pylori breath test is one. This test is separate from the hydrogen breath test because of what you ingest during the test. While the hydrogen breath test measures your digestion of sugars or carbohydrates, the carbon breath tests require you to consume a solution of carbon molecules. The two tests are used separately to diagnose different gastrointestinal diseases.

What are the different types of hydrogen breath tests?

When you take a hydrogen breath test, you’ll give a breath sample before and several after consuming a particular test sugar — usually lactose, fructose, sucrose, sorbitol, glucose or lactulose. The choice of sugar depends on what kind of condition your healthcare provider is checking for.

  • Lactose, fructose, sucrose and sorbitol tests check for intolerance to those particular sugars. These are all sugars that, for different reasons, can be hard for some people to digest. You’ll only be tested for one at a time. If the breath test results in high levels of hydrogen, it means that sugar wasn’t well digested in your intestines.
  • Glucose is the preferred sugar to test for small intestinal bacterial overgrowth (SIBO). That’s because glucose is usually very quickly absorbed in your small intestine. If glucose is being fermented by anaerobic bacteria and producing hydrogen, it means that too many of those bacteria have moved into your small intestine, and they’re digesting the glucose before it has had a chance to be absorbed.


How do I prepare for the hydrogen breath test?

Certain foods, medications and activities can alter the results of the hydrogen breath test. Your healthcare provider will ask you to follow some specific guidelines in the weeks, days and hours leading up to the test to maximize its accuracy.

  • One month before: You’ll be asked to stop taking antibiotics and probiotics, both of which can alter the normal balance of bacteria in your gut.
  • One week before: You’ll be asked to stop smoking. Also stop taking any laxatives or fiber supplements, antacids and prokinetics, which affect your motility.
  • One day before: You’ll be asked to limit your diet to only low-fiber, easily digested foods. Specific recommendations may vary, but the diet often excludes oils and fats, sweeteners and seasonings, dairy products and grains, except for white bread and white rice. Baked or boiled fish or poultry with salt and pepper are safe bets.
  • On the day of the test: You’ll be asked to fast for 12 hours. That means no food or water. You’ll also be asked to avoid exercising or sleeping within a few hours of the test.

What should I expect during the hydrogen breath test?

You’ll give your first breath sample by breathing into a breathalyzer machine, which often looks like an inflatable bag with a tube attached. Then, you’ll drink the sugar solution. After consuming the sugar, you’ll continue to give breath samples for the next few hours, at intervals of about 15 to 30 minutes. You can watch TV or read while you’re waiting between samples. Physical activity should be limited during the test. You may also be asked to record your digestive symptoms resulting from the test sugar — any abdominal pain, bloating, farting, etc.

Are there any risks or side effects to the hydrogen breath test?

The hydrogen breath test is noninvasive and generally harmless, but it does identify digestive problems by inducing those problems. So while breathing into a bag might be painless in itself, ingesting the test sugar may well result in some abdominal discomfort — the same kinds of discomfort that made you seek medical care in the first place. Hopefully, the test will help isolate the underlying cause of your symptoms. Once you’re able to treat the cause, you won’t have to suffer from your symptoms much longer.

Results and Follow-Up

How do I interpret the results of my hydrogen breath test?

Hydrogen levels are measured in parts-per-million (ppm). Normal hydrogen values in a healthy digestive system are less than 16 ppm. If your baseline level is more than 16 ppm on the day of the test, you may be asked to go back on the pre-test diet and try again another day.

When the hydrogen breath test is given to test for carbohydrate malabsorption, a rise of more than 20 ppm over the baseline is considered a positive test. For SIBO testing, the rise in hydrogen by 20 ppm above the baseline must occur within 90 minutes, as hydrogen is being fermented in your small intestine instead of in your colon.

How accurate or reliable are results of the hydrogen breath test?

There are a few complicating factors that can muddle results from the test. If you didn’t follow the protocol leading up to the test — for example, if you took antibiotics or laxatives, or didn’t fast — your test results might not be accurate. If you’re one of those people (about 15% to 30%) whose gut bacteria tend to produce methane more than hydrogen, this may affect your results.

If you do have a positive result from the hydrogen breath test, you can be sure that the hydrogen resulted from poor digestion of the test sugar. Tests for specific sugar intolerances and the glucose test for SIBO are very reliable for diagnosing those conditions.

Sometimes, those conditions might not fully explain all of your symptoms, or treating those conditions might not fully cure your symptoms. Then, further testing might be needed to identify other factors causing them, or implement other dietary changes or treatments that might help improve them.

A note from Cleveland Clinic

Gastrointestinal symptoms are no fun. Discomfort from gas and indigestion can interfere with your enjoyment of everyday life, and may mean that your body isn’t getting the nutrients it needs. These symptoms can have many causes, but often, they’re relatively simple to diagnose and treat. A little information can go a long way on the path to recovery, and the hydrogen breath test is one of the easiest ways to obtain it.

Medically Reviewed

Last reviewed on 05/03/2022.

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