A lactose tolerance test is a test that measures how well your body can process lactose, a type of sugar found in dairy products. Normally the body is able to process milk and other dairy products thanks to an enzyme in the intestines called lactase, which breaks lactose down during digestion.

However, some people have levels of lactase that are lower than normal. For these people, instead of breaking down the milk sugar (lactose), the lactose will persist in the intestines and ferment. This inability to digest lactose is referred to as lactose intolerance.

There are three kinds of tests that can be used to check lactose intolerance:

Hydrogen Breath Test

This test is the most common. During this test, your doctor will instruct you to drink a liquid solution containing lactose, and will then have you breathe into a balloon-like instrument at set intervals to determine how much hydrogen there is in your breath. The more hydrogen you exhale, the more likely it is that your body is unable to process lactose.

Lactose Tolerance Test

Like the hydrogen breath test, this test also requires you to drink a liquid with lactose. After two hours, your doctor will take a blood sample to measure how much glucose is in your blood. If your blood glucose level does not rise, this means that your body is not digesting or absorbing lactose.

Stool Acidity Test

This test is usually performed on infants or small children, as they are not eligible for other tests. A physician will take a stool sample to see if lactose is breaking down properly in your system. Fermenting lactose in the intestines (a sign of lactose intolerance) creates lactic acid, which can be detected in stool.

These tests may be performed in your physician’s office or in an outpatient laboratory.

Your physician may recommend this test if you have symptoms of lactose intolerance. Signs of lactose intolerance include feeling nauseated or getting cramps shortly after consuming dairy foods. Diarrhea, gas, and bloating after eating foods with lactose are also common symptoms.

According to the National Digestive Diseases Information Clearing House (NDDIC), lactose intolerance occurs more often in older adults and babies who are born prematurely. If your doctor thinks you are lactose intolerant, he or she may ask you to eliminate all dairy products from your diet for a short time to see if symptoms improve.

To prepare for the test, it is a good idea to ask your doctor about which test you will be taking, and to ask about any dietary restrictions. Your physician may ask that you not eat or drink for eight hours before the test. Your physician may also restrict your exercise.

Make sure your provider knows about any other conditions you may have and any medications you are currently taking. Some medications and foods can interfere with test results. Smoking may also affect test results. If you are a smoker, ask your doctor if you should abstain from tobacco prior to taking the test.

Hydrogen Breath Test

Before the test, your doctor will ask you to breath into a balloon-like device to obtain a baseline, or typical reading of how much hydrogen is in your breath after fasting. Next, your doctor will have you drink a liquid that contains lactose and water. As your body digests the lactose, your doctor will ask you to breathe into the balloon-like device several times over a few hours to measure how much hydrogen is in your breath. This is typically done in 15-minute intervals for approximately two to three hours. Increased levels of hydrogen in the breath indicate that the body is not properly breaking down or absorbing lactose.

Lactose Tolerance Test

For this test, your doctor will take a fasting blood sample to obtain a baseline, or typical, reading of how much glucose is in your blood. Then, like the hydrogen breath test, your doctor will have you drink a liquid with lactose. As your body digests the lactose, it should break the lactose down into a simpler sugar called glucose. Your doctor will take blood samples several times over a few hours to measure how much glucose is in your blood.

Stool Acidity Test

This test is primarily for infants and young children. In this test, the doctor will have the infant or child drink a liquid with lactose. After waiting, the physician will take a stool sample. Normally, a stool is not acidic. However, if the body cannot break down lactose, lactic acid and other acids will appear in the stool.

Your doctor will discuss the results with you after the test. Remember, some results vary by doctor’s office and clinic.

Hydrogen Breath Test

A hydrogen breath test result that shows a rise in hydrogen of more than 12 parts per million over your original fasting test indicates lactose intolerance.

Lactose Tolerance Test

You may be lactose intolerant if blood glucose levels raise less than 20 mg/dL (milligrams per deciliter) within two hours of drinking the lactose solution, according to the U.S. National Library of Medicine.

Be sure to discuss what these results mean with your doctor and how you can manage your condition.


How can people who are lactose intolerant choose the right milk alternatives?



There are alternatives to dairy that include rice, soy, coconut, almond, and lactose-free milk. Keep in mind to read the labels because some of these alternatives have added sugar or a higher content of fat or triglycerides, depending on the source.

Mark LaFlamme, M.D.Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.