Learn About the Small Intestine Bacterial Overgrowth Breath Test

Should You Have a Bacterial Overgrowth Breath Test?

patient blowing into breath test device with doctor in background
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You may have a small intestine bacterial overgrowth breath test if you have symptoms of a disorder doctors call bacterial overgrowth syndrome -- excessive growth of bacteria in the small intestine that can make it harder for your body to absorb nutrients from the food you eat. This condition is called malabsorption.

Advanced (long-lasting) bacterial overgrowth syndrome can lead to such malabsorption-related health problems as anemia, dermatitis, and body wasting and weakness (cachexia).

It's important to diagnose and ​treat bacterial overgrowth syndrome as early as possible. For this, the small intestine bacterial overgrowth test is key.

What Happens in Bacterial Overgrowth Syndrome?

Although there are normally plenty of bacteria in the large intestine, the small intestine typically contains far fewer, because 1) stomach acid helps prevent bacteria from growing and 2) food is usually moved through rapidly. However, certain conditions may promote the growth of excess bacteria in the small intestine, including low levels of stomach acid, damage to the lining of the small intestine, and slow movement of food through it.

Who's Likely to Have the Bacterial Overgrowth Breath Test?

This test may be recommended for people with digestive disorders accompanied by abdominal bloating and pain, gas, diarrhea, indigestion, weight loss, and, in some people, problems digesting dairy products or other carbohydrates.


How Should You Prepare for This Test?

Here are some of the things you need to know.

The day before your test:

  • Eat only foods that the body absorbs well, including baked or broiled chicken or turkey (use nothing but salt and pepper on it), only white bread and rice, eggs, and clear chicken or beef broth.
  • Drink only water, tea, or unflavored black coffee.
  • Do not eat pasta, fruits, vegetables, nuts or foods with seeds, or dairy products, including cheese.

For 12 hours before your test, you need to fast (eat nothing and drink nothing but water).

There are other things you need to know. Your doctor should give you a complete list of what to do and not to do before you have your bacterial overgrowth breath test. 

How Is This Test Performed?

To start the test, you drink a beverage containing the sugar lactulose. If there is bacterial overgrowth in your small intestine, the lactulose ferments and produces the gases hydrogen and methane. Some of this gas enters your bloodstream and is brought to your lungs -- the reason why it's possible to test your breath for the bacterial overgrowth. 

Once fermentation has occurred, taking the bacterial overgrowth breath test is simple. You blow into a mouthpiece that collects your breath in a bag or vacuum-sealed collection tubes. This breath sample is then tested for abnormally high levels of hydrogen and methane.

In general, the higher your level of these gases, the more small-intestine bacterial overgrowth you have.

Disclaimer: The information contained on this site is intended for educational purposes only and is not a substitute for advice, diagnosis, or treatment by a licensed physician. It is not meant to cover all possible precautions, drug interactions, circumstances, or adverse effects. You should seek prompt medical care for any health issues and consult your doctor before using alternative medicine or making a change to your regimen.


“Hydrogen/methane breath tests.” JohnsHopkinsMedicine.Org (2016). 

Syed SZ. “Bacterial overgrowth syndrome clinical presentation.” Medscape.Org (2016).  

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