Hydrogen Breath Test

What is the hydrogen breath test?

The hydrogen breath test is a test that uses the measurement of hydrogen in the breath to diagnose several conditions that cause gastrointestinal symptoms. In humans, only bacteria - specifically, anaerobic bacteria in the colon - are capable of producing hydrogen. The bacteria produce hydrogen when they are exposed to unabsorbed food, particularly sugars and carbohydrates, but not proteins or fats. Although limited hydrogen is produced from the small amounts of unabsorbed food that normally reach the colon, large amounts of hydrogen may be produced when there is a problem with the digestion or absorption of food in the small intestine, that allows more unabsorbed food to reach the colon.

Large amounts of hydrogen also may be produced when the colon bacteria move back into the small intestine, a condition called bacterial overgrowth of the small bowel. In this latter instance, the bacteria are exposed to unabsorbed food that has not yet had a chance to completely traverse the small intestine to be fully digested and absorbed. Some of the hydrogen produced by the bacteria, whether in the small intestine or the colon, is absorbed into the blood flowing through the wall of the small intestine and colon. The hydrogen-containing blood travels to the lungs where the hydrogen is released and exhaled in the breath where it can be measured.

When is hydrogen breath testing used?

Hydrogen breath testing is used in the diagnosis of three conditions.

  • The first is a condition in which dietary sugars are not digested normally. The most common sugar that is poorly digested is lactose, the sugar in milk. Individuals who are unable to properly digest lactose are referred to as lactose intolerant. Testing also may be used to diagnose problems with the digestion of other sugars such as sucrose, fructose and sorbitol.
  • The second condition for which hydrogen breath testing is used is for diagnosing bacterial overgrowth of the small bowel, a condition in which larger-than-normal numbers of colonic bacteria are present in the small intestine.
  • The third condition for which hydrogen breath testing is used is for diagnosing rapid passage of food through the small intestine. All three of these conditions may cause abdominal pain, abdominal bloating and distention, flatulence (passing gas in large amounts), and diarrhea.

How does hydrogen breath testing work?

The bacteria in the colon, including the anaerobic bacteria, are able to digest and use sugars and carbohydrates as food. When the anaerobic bacteria digest sugars and carbohydrates, they convert some of the sugars and carbohydrates into gases, most commonly hydrogen. They also may produce and release into the colon other substances, for example, chemicals that cause the colon to secrete water and cause diarrhea.

As previously discussed, some of the hydrogen gas is absorbed by the colon into the blood and is eliminated in the breath where it can be measured. As long as little sugar or carbohydrate reaches the colon, the small amounts of gas and other substances that are produced do not cause a problem. When larger amounts of sugar or carbohydrate reach the colon because they are not digested and absorbed in the small intestine, larger amounts of gas and substances are formed in the colon.

For example, if an individual digests and absorbs the sugar in milk (lactose) normally, then none of the lactose that is given for the lactose hydrogen breath test reaches the colon, and no increase in the concentration of hydrogen in the breath is seen during the breath test. On the other hand, if the individual does not digest and absorb the lactose completely, that is, he or she is lactose intolerant, the lactose travels through the small intestine and enters the colon where the bacteria digest it and produce hydrogen. An increase in hydrogen in the breath then is seen. Other sugars for which poor digestion can be diagnosed by breath testing include sucrose and fructose (found in corn syrup), and sorbitol (a sugar that is used as a low-calorie sweetener).

There are ways other than abnormal digestion of dietary sugars by which the bacteria can cause problems. Unlike in the colon, the number of hydrogen-producing, anaerobic bacteria in the small intestine is small. If, however, large numbers of hydrogen-producing bacteria move into the small intestine from the colon, a condition called bacterial overgrowth of the small bowel, the bacteria may digest the sugars and carbohydrates before the small bowel has had a chance to digest and absorb them and produce large amounts of hydrogen.

Finally, if individuals have abnormally rapid passage of food through the small intestine, there may not be enough time for the small intestine to digest and absorb sugars and carbohydrates. This results in the entry of larger amounts of sugar and carbohydrate into the colon where the bacteria can digest and convert them to gas.

To diagnose bacterial overgrowth and rapid transit through the small intestine, a sugar that is not digested and absorbed by man, such as lactulose, usually is used for the test. In the case of rapid passage through the small intestine, the sugar passes quickly through the small intestine and into the colon so that hydrogen is found in the breath very soon after ingestion of the sugar. In the case of bacterial overgrowth, production of hydrogen occurs twice during the test. Once as the sugar passes the bacteria in the small intestine and again when the sugar enters the colon.


Bowel regularity means a bowel movement every day. See Answer

How is hydrogen breath testing performed?

Prior to hydrogen breath testing, the patient fasts for at least 12 hours. At the start of the test, the patient blows into and fills a balloon with a breath of air. The concentration of hydrogen is measured in a sample of breath removed from the balloon. The patient then ingests a small amount of the test sugar (lactose, sucrose, sorbitol, fructose, lactulose, etc. depending on the purpose of the test). Additional samples of breath are collected and analyzed for hydrogen every 15 minutes for up to five hours.

How are the results of hydrogen breath testing interpreted?

The interpretation of the results of hydrogen breath testing depends on the sugar that is used for testing, and the pattern of hydrogen production after the sugar is ingested.

After ingestion of test doses of the dietary sugars lactose, sucrose, fructose or sorbitol, any production of hydrogen means that there has been a problem with digestion or absorption of the test sugar and that some of the sugar has reached the colon.

When rapid intestinal transit is present, the test dose of non-digestible lactulose reaches the colon more quickly than normally, and, therefore, hydrogen is produced by the colonic bacteria soon after the sugar is ingested.

When bacterial overgrowth of the small bowel is present, ingestion of lactulose results in two separate periods during the test in which hydrogen is produced, an earlier period caused by the bacteria in the small intestine and a later one caused by the bacteria in the colon.

What are the limitations of hydrogen breath testing?

There are several limitations to hydrogen breath testing. For unclear reasons, testing for bacterial overgrowth of the small bowel with lactulose can diagnose only 60% of cases. (This insensitivity of the test may be due in part to the strict criteria that are used for interpreting a test as abnormal. Less strict criteria may diagnose overgrowth more often.) In addition, with bacterial overgrowth there may be an overlap of the early and later periods of hydrogen production that can be misinterpreted as a single period characteristic of rapid intestinal transit. Finally, some normal individuals may have slow transit through the small intestine making prolonged testing - up to 5 hours - necessary, and many individuals are unwilling to undergo such prolonged testing.

Some individuals do not have bacteria that produce hydrogen, and in these individuals hydrogen breath testing is not possible. Most of these individuals have bacteria that produce a different gas, methane. (There also are individuals who produce both hydrogen and methane.) Methane can be measured in the breath just like hydrogen, and the production of methane can be used for diagnosis in the same way as hydrogen. There is much less experience with methane, however, and the production of methane is more complex than the production of hydrogen. Therefore, it is not clear if the pattern of methane production after ingestion of sugars can be interpreted in the same way as hydrogen production, particularly for the diagnosis of bacterial overgrowth.

A pattern of hydrogen production that is typical for bacterial overgrowth does not necessarily mean that an individual's symptoms are caused by the overgrowth. For example, there may be anatomic abnormalities of the small intestine such as narrowing or functional abnormalities in the way the muscle of the small intestine works. These abnormalities can cause symptoms of bloating, distention, pain, and diarrhea themselves, but they also can lead to bacterial overgrowth with its similar symptoms. Therefore, it may be an underlying abnormality that is responsible for the symptoms and not the bacterial overgrowth. The only way to differentiate between the two causes of symptoms-an underlying problem or bacterial overgrowth - is to treat and eradicate the bacteria. If the symptoms disappear, then it is more likely that it is the overgrowth rather than the underlying abnormality that is responsible for the symptoms.

Any condition that results in the delivery of undigested or unabsorbed food to the colon may result in abnormal breath tests when dietary sugars are used for testing. Both pancreatic insufficiency and the condition called celiac sprue can cause abnormal breath tests, in the former instance because pancreatic enzymes that are necessary for the digestion of carbohydrates are missing, and in the latter instance because the lining of the small intestine is destroyed, and digested food cannot be absorbed. It may be necessary to exclude these causes of abnormal breath tests by additional tests-pancreatic function tests and small intestinal biopsy.

Are there other ways in which hydrogen breath testing can be used?

Antibiotics are used for treating bacterial overgrowth of the small bowel; however, any one antibiotic may be effective at eliminating the overgrowing bacteria only 50%-60% of the time. Therefore, if symptoms do not disappear in an individual following treatment with antibiotics, it may be useful to repeat the breath test to determine if the antibiotics have eliminated the bacteria. If not, a different antibiotic or non-antibiotic treatment can be tried.

What are the side effects of hydrogen breath testing?

The side effects of hydrogen breath testing are exactly what one would expect to see in individuals who poorly digest and absorb sugars and carbohydrates, for example, bloating, distention, pain, and diarrhea. When lactulose is used these symptoms are unlikely to occur or are mild because the dose of lactulose used for testing is small.

What are the alternatives to hydrogen breath testing?

For diagnosing lactose intolerance, an alternative procedure to breath testing requires blood samples to be taken after the ingestion of lactose. If the digestion and absorption of lactose is normal, the levels of glucose in the blood should rise. The elevation of blood glucose occurs because the lactose is broken down into its two component sugars, galactose and glucose, as it is absorbed into the blood. A second alternative is to give a dose of lactose (or other dietary sugar) and observe an individual for symptoms. If the individual is intolerant, bloating, distention, pain, flatulence, and diarrhea are likely to occur. A third alternative is a trial of a diet in which the potentially-offending sugar is strictly eliminated. All of these alternatives, however, have limitations and problems.

Bacterial overgrowth can be diagnosed by culturing (growing) the bacteria from a sample of fluid from the small intestine and counting the numbers of colonic bacteria that are present. This procedure requires a tube to be passed through the nose, throat, esophagus and stomach under X-ray guidance so that fluid can be obtained from the small intestine. It is an uncomfortable and expensive procedure, and most laboratories are not able to accurately culture the samples. Hence, this test is not performed routinely.

Medically reviewed by Avrom Simon, MD; Board Certified Preventative Medicine with Subspecialty in Occupational Medicine


United States. National Institutes of Health. "Use and abuse of hydrogen breath tests."