The hydrogen breath test helps doctors diagnose certain causes of gut symptoms.
It’s mainly used to diagnose small intestine bacterial overgrowth (SIBO) and lactose intolerance.
The test is relatively simple. Below, you’ll learn more about why doctors ask for it, how it works and what to expect.
What are hydrogen breath tests?
As bacteria in your digestive system break down sugars, they produce gases – including hydrogen.
The hydrogen breath test measures the amount of this gas to provide clues about what’s happening in your gut.
Often, a breath test also measures the amount of methane, another gas that gut microbes tend to produce.
The hydrogen breath test can help diagnose:
There are breath tests that check for intolerances to sugars other than lactose, like fructose. However, these aren’t routine in clinical practice because they aren’t very accurate.
We’ll explain how the test checks for SIBO and lactose intolerance in a bit more detail later on.
Beyond these two health issues, a hydrogen breath test may also help a doctor reach a diagnosis of irritable bowel syndrome (IBS). No single test points to IBS, but a hydrogen test can help by ruling out SIBO and lactose intolerance.
What happens during a hydrogen breath test?
At the start of the test, you breathe into a hand-held breathalyser, which looks like a tube attached to an inflatable bag.
Next, you drink a sugary solution and breathe into the breathalyser every 15–30 minutes for a few hours.
This shows the doctor how levels of gas in your breath change as your gut bacteria help digest the sugars.
Commercial, at-home hydrogen breath test kits are available, but many aren’t reliable. If your doctor has referred you for this test, you’ll be expected to take it in a hospital.
What happens before a breath test?
The test itself is relatively simple, but there are important steps before your appointment. For instance, there are certain foods you can’t eat.
When a doctor refers you for a hydrogen breath test, they’ll give you instructions. These may vary slightly between clinics, but they’re likely to include the following steps.
For 4 weeks before the test
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Don’t take antibiotics. If you’re already taking antibiotics, keep taking them – your doctor will rebook your test.
For 24 hours before the test
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Don’t eat foods that take a long time to digest, such as beans, bran and cereals.
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Your doctor might ask you to only eat easily digested foods, like scrambled eggs, plain toast, rice, chicken or pasta.
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Don’t take fibre supplements.
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Don’t drink milk.
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Don’t have pickled vegetables or fermented foods.
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Don’t take any laxatives.
For 12 hours before the test
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Don’t eat or drink anything (including water and chewing gum).
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If you need to swallow prescribed tablets, try to take them with the smallest amount of water possible.
For 30 minutes before the test
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Don’t smoke, and avoid smokey places.
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Don’t sleep.
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Don’t exercise.
Who needs a hydrogen breath test?
To diagnose SIBO and lactose intolerance, doctors use different breath tests.
SIBO breath test
In healthy people, most gut bacteria live in the large intestine, towards the end of the digestive tract.
People with SIBO have an overgrowth of bacteria in the small intestine, which is just below the stomach. This overgrowth can interfere with digestion.
The body typically digests and absorbs most sugars before they reach the microbes in the large intestine. But an overgrowth of bacteria can break down these sugars in the small intestine, producing gases such as hydrogen.
As a result, people with SIBO experience symptoms like excess wind, bloating, pain, diarrhoea and feelings of fullness.
You consume glucose during the breath test for SIBO because this simple sugar is readily absorbed in the small intestine.
If you breathe out high levels of hydrogen, it means that bacteria in the small intestine are breaking down the glucose before it has a chance to be digested and absorbed.
Lactose intolerance
Some people have difficulty digesting lactose, a sugar in dairy products. This issue is called lactose intolerance.
If your body has trouble breaking down lactose, gut bacteria feed on it instead, producing gas.
If a breath test detects high levels of hydrogen after you consume lactose, it shows that your gut bacteria are breaking down this sugar, rather than your digestive enzymes.
How does hydrogen get into your breath?
As your gut bacteria feed, the gasses they produce travel through the lining of your gut and enter your bloodstream.
Once the gasses reach your bloodstream, your circulatory system carries them throughout your body. Ultimately, the gasses enter your lungs, and you breathe them out.
Are breath tests safe?
Hydrogen breath tests are safe, even during pregnancy and breastfeeding.
However, after you drink the sugar solution, you may experience the symptoms that caused you to take the test in the first place.
For some people, the sugar solution might also cause diarrhoea.
What does the test show?
By assessing the level of hydrogen in your breath, doctors can diagnose SIBO or lactose intolerance. However, it’s not always an open-and-shut case.
For instance, if you aren’t able to follow the instructions before the test, it can throw off the results.
Also, some gut microbes produce higher levels of methane than hydrogen, which can affect the results. Because populations of gut microbes vary so much from person to person, most breath tests measure both hydrogen and methane.
Like most diagnostic tests, hydrogen breath tests sometimes produce false positives. In other words, the results suggest that you have a health condition when you don’t. This may be most common among commercial kits, which tend to be unreliable.
It’s also important to note that other gut conditions can accompany issues like lactose intolerance.
If your test points to lactose intolerance, and you cut out lactose from your diet, it’s possible that you may still experience gut symptoms. In this case, you’ll need to go back to your doctor.
Going on an elimination diet (under the guidance of a dietitian) can help diagnose lactose intolerance without the need for breath testing.
Summary
Doctors use the hydrogen breath test to diagnose SIBO and lactose intolerance. The test can also rule out these conditions, which may help a doctor diagnose IBS, for example.
While there are many instructions to follow before your test, the procedure itself is simple. After you drink a sugary solution, a doctor measures changes in the levels of hydrogen, and often methane, in your breath.
To get the most accurate results, it’s important to follow the steps to prepare for the test.
Hydrogen breath tests are safe, though they may cause digestive symptoms. And like most diagnostic tests, they aren’t 100% accurate.
To learn more about other gut health topics, visit Symprove’s Gut Hub.
Sources
Hydrogen breath test. (2019). Link.
Hydrogen breath test. (n.d.). Link.
Pros and cons of breath testing for small intestinal bacterial overgrowth and intestinal methanogen overgrowth. Gastroenterology and Hepatology. (2023). Link.
