What Is a Lactose Tolerance Test?

A lactose tolerance test evaluates how effectively your body can break down lactose, the sugar present in milk and other dairy items.

Under normal circumstances, an intestinal enzyme called lactase enables the digestion of milk and dairy by splitting lactose into absorbable sugars.

Some people produce less lactase than typical. In these cases, lactose remains in the gut and ferments. This reduced ability to digest lactose is known as lactose intolerance.

Types of tests

There are three principal tests used to determine lactose intolerance.

Hydrogen breath test

This is the most frequently used test. During the procedure, your clinician will have you drink a liquid that contains lactose. At scheduled times you’ll exhale into a balloon-style device to measure the hydrogen concentration in your breath.

The higher the hydrogen level you blow out, the more likely it is that your body is not properly digesting lactose.

Lactose tolerance test

Similar to the hydrogen breath method, this test involves drinking a lactose-containing liquid. Two hours later, a blood sample is taken to assess your blood glucose level.

If your blood glucose does not increase, it indicates your body is not digesting or absorbing lactose effectively.

Stool acidity test

This test is typically used for infants and very young children who cannot undergo the other tests. The clinician will analyze a stool sample to determine whether lactose is being broken down correctly.

When lactose ferments in the intestines — a hallmark of lactose intolerance — it produces lactic acid, which can be detected in stool.

These evaluations can be performed in a physician’s office or at an outpatient laboratory.

When would your doctor recommend a lactose tolerance test?

Your clinician may suggest this testing if you show signs of lactose intolerance.

Common symptoms people experience after eating or drinking dairy include:

• nausea
• abdominal cramps
• diarrhea
• excess gas
• bloating

Lactose intolerance is more common in older adults and in infants born prematurely.

If your doctor suspects lactose intolerance, they might advise removing dairy from your diet temporarily to see whether your symptoms lessen.

How do you prepare for a lactose tolerance test?

Before the test, ask your doctor which specific test you will have and whether there are any dietary or activity restrictions. Often you’ll be asked not to eat or drink for about 8 hours prior to testing. Your clinician may also limit physical activity before the test.

Tell your provider about any medical conditions and all medicines you take. Certain drugs and foods can alter test outcomes.

Smoking can also influence results. If you smoke, check with your physician about refraining from tobacco before the test.

What happens during a lactose tolerance test?

Hydrogen breath test

Initially, your clinician will ask you to breathe into a balloon-like instrument to get a baseline hydrogen reading after fasting.

You will then drink a lactose-containing solution. As digestion proceeds, you’ll breathe into the device at several points over a few hours so hydrogen levels in your breath can be measured.

This is commonly done every 15 minutes for about 2 to 3 hours. Elevated hydrogen in the breath suggests the body is not adequately breaking down or absorbing lactose.

Lactose tolerance test

For this test, a fasting blood sample is taken to determine your baseline blood glucose. After you consume the lactose solution, your body should convert lactose into the simpler sugar glucose. Multiple blood samples are taken over several hours to track glucose changes.

Stool acidity test

This method is mainly for infants and small children. The child will ingest a lactose solution, and after a period the clinician will collect a stool sample.

Typically, stool is not acidic. If lactose isn’t being digested, lactic acid and other acids will be present in the stool.

Interpreting your test results

Your clinician will review the results with you after testing. Keep in mind reference ranges and interpretation may vary between clinics.

Hydrogen breath test

A rise in breath hydrogen of more than your baseline reading indicates lactose intolerance.

Lactose tolerance test

If blood glucose increases by less than 20 milligrams per deciliter (mg/dL) within 2 hours of drinking the lactose solution, you may be lactose intolerant.

Discuss the implications of your results and management options with your doctor.