Lactose intolerance is an inability to digest a type of sugar found in milk and dairy products. Breath, blood, and stool tests are types of lactose intolerance tests that can measure the body’s ability to break down lactose.

In the United States, around 36% of the population has lactose intolerance, estimates the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Most people manage their symptoms by reducing or eliminating all lactose from their diet. While a formal diagnosis is not necessary, lactose intolerance tests can help confirm that the symptoms are because of lactose intolerance.

In this article, we explain how doctors test for lactose intolerance, some simple ways to test at home, and how to manage the symptoms of lactose intolerance, also known as lactose malabsorption.

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According to the NIDDK, doctors may suggest a person with suspected lactose intolerance stop drinking milk and eating milk products for a few days to see if the digestive problems go away.

Doctors will also want to rule out other conditions that have similar symptoms, including celiac disease, irritable bowel syndrome, and inflammatory bowel disease.

They will probably conduct a physical exam to check for bloating or tenderness in the abdomen. They may ask about family history, as lactose intolerance tends to run in families.

Types of lactose intolerance tests include hydrogen breath tests, blood glucose tests, and stool acidity tests.

For this test, a person with suspected lactose intolerance drinks a liquid that contains a known amount of lactose. They then breathe into a balloon-like container that measures the amount of hydrogen in the breath.

The NIDDK explains that everyone has a small amount of hydrogen in their breath. But people who have difficulty digesting lactose would have higher amounts of hydrogen.

An expert consensus in the American Journal of Gastroenterology says that before taking this test, most doctors will recommend:

  • fasting (not eating or drinking) for between 8–12 hours
  • avoiding antibiotics for up to 4 weeks before the test
  • not eating fermentable foods, such as complex carbohydrates or dairy products, on the day before the test
  • Avoiding laxatives and promotility medications for at least one week, if tolerated

Doctors may also ask people taking the hydrogen breath test not to smoke, and to limit their physical activity on the day before and day of the test.

At the start of the hydrogen breath test, doctors collect a breath sample to establish a baseline for the hydrogen in the person’s breath. They measure the amount of hydrogen in parts per million (ppm).

As described in the above consensus, the doctor tests the person’s breath at regularly spaced intervals for the next 3–4 hours. If the level of hydrogen in the breath increases by more than 20 ppm, it means the person likely has lactose intolerance.

Someone who suspects lactose intolerance can contact a healthcare professional for the hydrogen breath test. Home test kits are also available for purchase.

As the body digests lactose, it produces glucose. A person who has difficulty digesting lactose would have less glucose in their blood. With a glucose blood test, a medical professional takes blood samples and measures the amount of glucose in a person’s blood. Another name for this test is a lactose tolerance test.

Blood glucose testing instructions include:

  • fasting (not eating or drinking) for between 8–12 hours
  • avoiding antibiotics for up to 4 weeks before the test

For a blood glucose test, a healthcare professional inserts a needle into the person’s arm and takes a small blood sample to establish a baseline for the glucose levels.

The person taking the test drinks a liquid containing a known amount of lactose. Doctors take more blood samples at regular intervals, usually after 30 minutes, 1 hour, and 2 hours.

Doctors evaluate the level of glucose in the blood. As the National Health Service explains, if the level of glucose stays the same or rises slightly, it may indicate that the person is not digesting the lactose. But the North American Consensus on hydrogen and methane-based breath testing maintains that “tests of blood glucose levels following lactose ingestion depend on glucose metabolism and are not reliable.”

The stool acidity test for lactose intolerance measures the acidity, or pH, of a stool sample. Stools with a low pH indicate high acidity. A low-pH stool test result is a sign of carbohydrate malabsorption, but it is not specific to lactose.

The most common lactose intolerance test is the hydrogen breath test. But this type of lactose intolerance test may not be suitable for infants. Doctors may use the stool acidity test for babies and young children as an alternate method of assessing lactose intolerance, along with a trial of eliminating dairy from the diet.

Hydrogen breath lactose intolerance tests for home use are available for purchase online or over the phone. But there are some simple ways to help indicate if a person has trouble digesting lactose, according to the Academy of Nutrition and Dietetics.

Keeping a food diary: Make a note of everything the person eats or drinks for a week, including the time. Include all snacks and beverages. Also, make a note of when they experience any symptoms, remembering that people who have lactose intolerance usually react within a few hours of eating.

Eliminating all milk and dairy products: Cut out all milk, ice cream, yogurt, cottage cheese, and other dairy products for a few days and see if the symptoms disappear. As part of an elimination diet, slowly start to reintroduce small quantities of each particular type of dairy food back into the diet. If symptoms reappear, it might show which food or foods are triggering symptoms.

Processed and packaged foods may contain additional lactose. It is also in some prescription medicines and over-the-counter remedies, so a food diary and elimination diet should account for these items. A healthcare professional, such as a dietitian, can help manage the safety of an elimination diet, particularly for people who take medications and dietary or herbal supplements.

A person who is lactose intolerant may experience bloating, abdominal cramps, flatulence and burping, loud bowel sounds, and diarrhea after eating or drinking a milk product. For children, this includes breast milk. The NIDDK states that most people experience these symptoms within a few hours.

If, after eliminating lactose-rich foods from their diet, a person continues to experience symptoms, a doctor may recommend further tests. When the doctor can diagnose an underlying cause, such as infection, treatment of the condition may increase tolerance to lactose, according to the NIDDK.

There are no risks associated with hydrogen breath tests for lactose intolerance. The only risk to people taking a hydrogen breath test is that they may experience symptoms of lactose intolerance as a result of consuming lactose for the test.

According to a review in the journal Nutrients, if a person experiences symptoms after consuming the lactose sample, doctors can use this as evidence of lactose intolerance. This is especially important if a person receives a negative hydrogen breath test result.

People who have blood glucose tests may experience pain or bruising at the puncture site.

Lactose intolerance is a low-risk condition. Most people who have difficulty digesting lactose can manage their symptoms by reducing the amount of lactose they consume, based on NIDDK treatment recommendations. There are also products containing lactase, the enzyme that breaks down lactose. Taking lactase may reduce the chance of experiencing symptoms.

The Academy of Nutrition and Dietetics also recommends looking at alternative sources of calcium and vitamin D, both nutrients found in dairy products. Many supermarkets stock lactose-free versions, which most people can tolerate, but the Academy of Nutrition and Dietetics suggests eating more leafy green vegetables, almonds, and canned sardines, and drinking fortified orange juice.

Lactose intolerance in adults can cause bloating, flatulence, and diarrhea, usually within a few hours of eating or drinking a dairy product containing lactose.

Doctors can confirm a lactose intolerance diagnosis using hydrogen breath tests or measuring the blood glucose of a person after they consume a known amount of lactose. Lactose intolerance is manageable by eliminating or reducing dairy products in the diet.

Infants and young children with lactose intolerance may need lactose-free alternatives to milk or formula. The American College of Gastroenterology advises that a doctor or dietitian recommend suitable replacements.