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Lactose intolerance

Updated 12 October 2018

Understanding lactose-tolerance testing

A lactose tolerance test is a test that measures how well your body can process lactose, a type of sugar found in dairy products.

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A doctor may recommend a lactose-tolerance test if you suspect a problem digesting lactose from dairy products.

Symptoms of lactose intolerance may include abdominal pain, bloating, gas and diarrhoea.

The various tests 

If lactose intolerance is suspected, a lactose-tolerance test, hydrogen-breath test or in the case of a child a stool-acidity test will be carried out. 

Hydrogen breath test 

A hydrogen breath test is the most common procedure, the American Association for Clinical Chemistry says.

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.

Breath levels of hydrogen increase after you are given a lactose-loaded drink. If the samples have high amounts of hydrogen, it is likely that you are lactose-intolerant.

Glucose blood test 

A second procedure, a glucose blood test, evaluates whether blood-sugar levels increase with the consumption of lactose. If there is not a significant increase and a person has the symptoms described above, it's probably due to lactose intolerance.

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.

Care must be taken when interpreting results of this test in people with diabetes, the association warns.

lactose tolerance

Stool acidity test 

This test is only 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.

Interpreting your test results

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 rise less than 20mg/dL (milligrams per decilitre) within two hours of drinking the lactose solution, according to the US National Library of Medicine.

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

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