Lactose intolerance is usually diagnosed based on symptoms, but a healthcare provider can also perform breath and blood testing. Breath tests are also available as over-the-counter home test kits.

Lactose is a type of sugar found in milk. Some people lack enough of the lactase enzyme that breaks down lactose. People who do not produce enough lactase may have symptoms after eating dairy products that include diarrhea and abdominal pain.

Testing Available for Lactose Intolerance

Symptoms after eating or drinking dairy products that resolve when you avoid them can be enough to diagnose lactose intolerance. A healthcare provider can diagnose lactose intolerance based on this history, and people often self-diagnose the condition.

Some people might need or want to be formally tested. This is especially true when it will be helpful to rule out (or rule in) lactose intolerance as a cause of symptoms. Discuss the need for this testing with a healthcare provider.

A breath test or a blood test can be used if testing is done. Uncommon tests include a biopsy from the small intestine (a sample is collected and analyzed in the lab) or a genetic test.

Hydrogen Breath Test

A healthcare provider may perform a hydrogen breath test. This test measures hydrogen in your breath before and after a dose of lactose-containing beverage. Undigested lactose is broken down by bacteria in your gut, creating hydrogen gas and causing higher levels of hydrogen in the breath.

Preparation:

• You must not take antibiotics for two to four weeks before the test.
• Do not smoke or engage in vigorous exercise the day before and the morning of the test.
• Do not eat or drink anything for eight to 12 hours before the test.

Steps to perform the test:

1. Breathe into a balloon-like device that measures hydrogen.
2. Drink a solution that contains lactose.
3. Every 30 minutes for a few hours, breathe into the measuring device again.

A hydrogen level over 20 parts per million could indicate lactose intolerance. Any signs or symptoms will also be recorded during the test, as lactose-intolerant people may have abdominal pain or diarrhea.

Lactose Tolerance Glucose Blood Test

This test helps diagnose lactose intolerance because the body normally breaks lactose down into glucose in the blood. If you aren’t breaking down lactose, your blood glucose level won’t rise after being given a lactose-containing drink.

The same preparation as the breath test is needed (fasting and refraining from antibiotics, smoking, and vigorous exercise).

Steps in the test:

1. Blood is drawn to be measured for its glucose level.
2. The person drinks a beverage with a known amount of lactose.
3. Blood is taken two hours after drinking lactose and measured for glucose level.

If the glucose level increases by less than or equal to 20 milligrams per deciliter (mg/dL), it could mean that the person has lactose intolerance.

When the glucose level rises by 30 mg/dL or more, the body likely has broken down the lactose and the person doesn’t have lactose intolerance.

How to Use a Home Test Kit

Home test kits for lactose intolerance are also available without prescription. These kits are similar to the hydrogen breath test performed at healthcare facilities. A home test compares breath hydrogen before and after drinking a lactose-containing beverage.

This test is completed at home and then sent to a lab for analysis. The test kit contains test tubes, a straw, a powder or liquid solution containing lactose, and a return box or envelope.

Steps in home testing may include:

1. Follow instructions for preparation, such as fasting and refraining from antibiotics, smoking, and vigorous exercise.
2. Uncap a test tube and use the straw to breathe into the test tube. The cap should be quickly placed back on the tube. This is the baseline that will be compared to the breath samples taken after drinking lactose.
3. Drink a solution containing lactose. This might be a liquid or a powder mixed into water.
4. Use the provided straw to breathe into a test tube at the stated intervals (usually three).
5. Return the test kit to the lab using the box or envelope provided.
6. The results are measured and then communicated back to you.

A home test kit may not be covered by health insurance. Check with your insurance provider.

For Infants and Children

Most infants can digest lactose. Rarely, an infant will have what’s called primary lactase deficiency and will be unable to digest lactose. These babies will need special formula as human milk contains lactose.

Discuss your child’s symptoms with a healthcare provider to determine what testing is appropriate and whether the symptoms are due to lactose intolerance, milk allergy, or another condition. Your provider may recommend a lactose-free diet followed by slowly reintroducing lactose-containing foods back into the diet and monitoring your symptoms. Hydrogen breath testing can be done.

As babies grow into toddlers, their bodies may have trouble digesting lactose. This is called acquired lactose intolerance. It’s thought that almost 70% of adults have lactose intolerance. The ethnicities that are more likely to have lactose intolerance include:

• African American
• Asian American
• Hispanic/Latino
• Native American

Infants can also develop a temporary type of lactose intolerance. Secondary lactase deficiency can happen after an infection with rotavirus or giardia. These infections can damage the small intestine and lead to lactose intolerance. The problem reverses when the infection resolves.

A Word From Verywell

If you suspect your infant or toddler has lactose intolerance, a pediatrician will usually start by asking about your child’s symptoms, diet, and medical history. They may also recommend a temporary dairy-free diet to confirm lactose intolerance. Further testing, such as a breath or blood test, may be warranted in some cases.  

What Results Mean

The treatment for lactose intolerance is avoiding dairy products. However, dairy is a source of nutrients, including calcium and vitamin D, which are needed for bone health and other body functions. If you avoid dairy products, you will need to get those nutrients in another way, through other foods or supplements.

It’s possible to find dairy products with lactose removed or take a lactase supplement immediately before the first sip or bite of lactose-containing foods or drinks. Lactase is an enzyme that can help break down lactose, which might help avoid symptoms.

Next Steps After Diagnosis

People who need to or who decide to omit all dairy products from their diet should work with a healthcare provider to learn how to get enough calcium and vitamin D. Ideally, this would be a dietitian who can help create a healthful eating plan.

A long-term calcium and vitamin D deficiency can lead to bone-thinning conditions such as osteopenia or the more advanced osteoporosis.

Some people may be able to tolerate a certain amount of lactose without symptoms. Many people who are lactose intolerant can have 1 cup of milk, which contains about 12 grams of lactose, without significant problems.

Not all dairy products contain high levels of lactose. Certain foods, such as yogurt and hard cheeses, have lower levels of lactose and may be less likely to cause symptoms.

Summary

Lactose intolerance is usually diagnosed by symptoms after eating or drinking dairy products. Breath or blood testing is often not needed but may be performed.

People who are lactose intolerant and need to avoid dairy should work with a healthcare provider to ensure they are getting enough calcium and vitamin D. Lactose-reduced dairy products and lactase supplements can also be used.