The H2 breath test - also known as the hydrogen breath test - is a painless and less stressful diagnostic procedure. It is used to detect and assess various syndromes of the gastrointestinal tract. In particular, the H2 breath test can be used to detect food intolerances such as
can be detected and assessed.
The patient takes a special test solution before the test. It contains either lactose or fructose, and possibly also the sugars
- glucose,
- lactulose or
- sucrose.
If the test solution is not properly digested and metabolized in the intestine, the intestinal bacteria produce hydrogen (H2). The hydrogen first enters the bloodstream and is then exhaled through the lungs.
In the H2 breath test, the doctor measures the hydrogen concentration in the patient's exhaled air before and after administering the solution.
The H2 breath test is used to diagnose and assess syndromes and diseases of the gastrointestinal tract.
The H2 breath test can be used to detect, among other things
- food intolerances such as the relatively common lactose intolerance or fructose intolerance and
- a bacterial colonization of the small intestine
can be diagnosed. The symptoms that can be clarified with the help of an H2 breath test include

The H2 breath test can be used to analyze the hydrogen concentration in the exhaled air © MarekPhotoDesign.com | AdobeStock
To carry out an H2 breath test, the patient must be fasting. This means that they should not eat anything heavy for around 24 hours before the H2 breath test or around 12 hours beforehand.
- not eat anything at all,
- not consume any alcoholic drinks and
- only drink tap water or non-carbonated water.
In addition, the patient must not smoke in the 12 hours before the H2 breath test and only brush their teeth with water.
The patient should also not have taken any antibiotics or had a colonoscopy in the 4 weeks prior to the H2 breath test.
In the H2 breath test, the patient drinks a special test solution on an empty stomach. Depending on what is to be tested, it contains either
- Lactose (approx. 50 grams),
- fructose (approx. 25 grams),
- glucose (approx. 80 grams) or
- other sugars.
The patient must then blow into a special measuring device at defined intervals - approximately every 10 to 15 minutes. The device measures the hydrogen concentration in their breath. In addition, blood glucose can be measured at certain intervals during the H2 breath test.
During the entire H2 breath test, attention is also paid to whether the patient has symptoms such as
- flatulence,
- abdominal cramps,
- abdominal pain or
- diarrhea
develops. The H2 breath test takes about 2 to 4 hours in total.
In an H2 breath test, the concentration of hydrogen in the exhaled air is measured. The unit used is parts per milliion (ppm).
The value should be measured once before drinking the test solution and should be below 20 ppm. If the value rises above 20 ppm after drinking the test solution, the result is meaningful. This can be used to diagnose lactose intolerance, for example.