Hydrogen breath tests are a diagnostic tool that doctors use to identify small intestinal bacterial overgrowth (SIBO), lactose intolerance, and other digestive conditions. The test measures the amount of hydrogen — and sometimes other gases — in a person’s breath.

The test requires a person to drink a sugar solution. The type of sugar a doctor provides will depend on the condition they are trying to identify.

After some time has passed, the person begins collecting breath samples. Based on the types and amounts of hydrogen in a person’s breath, a doctor may be able to make a diagnosis.

Keep reading to learn more about the uses, types, preparation, steps, and results of a hydrogen breath test.

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Hydrogen breath tests are diagnostic tests that doctors use to identify certain digestive conditions.

Usually, very little hydrogen is present in a person’s breath. However, when bacteria in the small or large intestine ferment undigested sugars, the process produces several gases — including hydrogen.

When this happens, some of the hydrogen in the digestive tract enters the bloodstream. Blood carries it to the lungs, which release the gas via exhalation.

Hydrogen breath tests involve intentionally giving someone a type of sugar to see how the body responds. If there is an issue with digestion, bacteria in the intestines may produce hydrogen, which doctors then measure by taking breath samples.

Depending on the condition they are testing for, doctors may also test for other gases in the breath.

A doctor may recommend one of these tests if they suspect:

  • SIBO: In SIBO, microorganisms that usually live in the large intestine migrate to the small intestine and begin to grow in large numbers. This can result in bloating, gas, diarrhea, or constipation.
  • Carbohydrate malabsorption: People with carbohydrate malabsorption have difficulty breaking down and absorbing sugars in food. Lactose intolerance is one type of carbohydrate malabsorption, but people can also have issues digesting the fructose in fruit, or other sugars.
  • Motility issues: Digestive motility is the rate at which food moves through the small intestine. Hydrogen breath tests can help diagnose fast or slow motility by revealing how quickly sugars reach the large intestine.

There are several types of hydrogen breath tests. The one doctors choose will depend on what they are trying to diagnose.

Lactose hydrogen breath test

The lactose breath test reveals whether a person has lactose intolerance. The condition occurs when an individual has little or no lactase — the enzyme that breaks down lactose in the small intestine.

Fructose hydrogen breath test

The fructose breath test determines if a person may have difficulty digesting fructose, a sugar present in fruits and honey.

Lactulose hydrogen breath test

Lactulose breath tests diagnose SIBO. They involve giving a person a type of sugar the body cannot digest. This means the sugar travels all the way through the digestive tract until it reaches a person’s gut flora, which can break it down.

Usually, this does not happen until food enters the large intestine. But in people with SIBO, lactulose fermentation starts sooner due to microbes in the small intestine. They may produce hydrogen, methane, or hydrogen sulfide, depending on the type of overgrowth a person has.

Lactulose breath tests can also show doctors if a person has motility problems. If hydrogen levels spike earlier than they should, this may mean food has entered the large intestine too fast, and vice versa.

Glucose hydrogen breath test

A glucose hydrogen breath test can sometimes show whether a person has SIBO. However, this test has some drawbacks.

Glucose is the smallest and simplest type of sugar molecule. Most people absorb glucose quickly in the small intestine.

If bacterial overgrowth occurs in this part of the small intestine, the test will detect hydrogen or methane gases. But if the overgrowth is further down the digestive tract, the glucose might not reach it successfully. This could produce a false negative.

However, on average, glucose breath tests may be more sensitive than lactulose tests.

Hydrogen breath tests require some preparation. This involves a person:

  • fasting overnight
  • avoiding eating carbohydrates that the body digests slowly, such as potatoes and bread
  • avoiding exercising or smoking before the test

Individuals may also need to stop taking their medications for up to several weeks before they can have the test. This is true if they currently take:

People should speak with a doctor about any medications they are taking before they schedule the test. Individuals should not discontinue medications without consulting a doctor first.

The exact procedure for hydrogen breath tests can vary depending on the type of test a person receives. It generally involves the following steps:

  1. A person brushes their teeth thoroughly, uses an antiseptic mouthwash, and then rinses the mouth with water. This prevents bacteria in the mouth from affecting the test results.
  2. Next, the person will collect a baseline breath sample. This shows a doctor what a person’s hydrogen or methane levels are without the influence of sugar. To do this, a person breaths out forcefully into a balloon or breath collecting device.
  3. The next task is drinking the sugar solution. A person may need to clean their teeth again afterward.
  4. After a certain amount of time, the person begins collecting more breath samples at regular intervals. This often continues for up to 3 hours.
  5. A medical professional will record the results.

The sugar solutions doctors use in hydrogen breath tests can sometimes trigger symptoms. Otherwise, the test is noninvasive and safe.

Breath test results do not give a straightforward “positive” or “negative” result. They simply record how much of a gas is in a person’s breath. To distinguish a positive from a negative, doctors consider:

  • how soon hydrogen or methane begin to show on the test
  • how much they increase compared to a person’s baseline
  • if, when, and how often the gas levels peak

The guidelines doctors tend to follow in the United States are as follows:

ConditionTestPositive result
SIBOglucose or lactulose breath test• hydrogen increase of at least 20 parts per million (ppm) above baseline within 90 minutes
• methane increase of 10 ppm or more above baseline
• a double peak
Lactose intolerancelactose breath testhydrogen increase of 20 ppm or more above baseline at any point during the test
Fructose malabsorptionfructose breath testhydrogen increase of 20 ppm or more above baseline at any point during the test
Motility issueslactulose breath test• late or no peak for slow transit time
• early peak for fast transit time

However, it is worth noting that there is no global consensus on whether these thresholds are correct.

Some researchers note that it can be difficult to interpret the results of these tests. For example, if a test shows an early peak in hydrogen, this could either be evidence of fast motility or SIBO.

Doctors may need to combine hydrogen breath tests with other tests to make a firm diagnosis.

Below are answers to some frequently asked questions about hydrogen breath tests.

Can you do hydrogen breath tests at home?

Some companies make at-home hydrogen breath test kits. A person completes the test at home and then sends their breath samples to a laboratory for analysis.

This option can be convenient, but there is a risk that a person will make a mistake. Completing the test at a medical facility means help is available if a person requires assistance collecting the samples.

How long do hydrogen breath tests take?

They can take up to 3 hours to complete.

How much do hydrogen breath tests cost?

The cost varies depending on whether a person has health insurance. Some policies may cover costs, but people should check with their health insurance company first.

A person usually has very little hydrogen in their breath. However, people with certain digestive conditions can have elevated hydrogen levels in their breath. This happens when gut microbes ferment undigested sugars in the intestines, which produces gases.

Hydrogen breath tests measure how much hydrogen a person exhales as a result of this process. Drinking different types of sugar can help diagnose different conditions. For example, a test with lactose can detect lactose intolerance, and a test with fructose can detect fructose malabsorption.

The type of sugar a doctor provides for the test will depend on the condition they are trying to identify.