At a Glance

Why Get Tested?

To help diagnose lactose intolerance in individuals who have difficulty digesting dairy products, or sometimes as part of an investigation of malabsorption, which is difficulty digesting food and absorbing nutrients from food

When To Get Tested?

When you have symptoms such as abdominal pain, bloating, gas, and diarrhea after consuming milk and other dairy products

Sample Required?

A series of breath samples exhaled into a collector, or a series of blood samples drawn from a vein in your arm

Test Preparation Needed?

Overnight fasting is required; nothing but water is permitted. Two weeks prior to the test, you must be off antibiotics and stomach medications, such as laxatives, antacids and stool softeners. Tell your healthcare practitioner about any other medications you are taking. Avoid strenuous activities. You may be instructed to brush your teeth and/or rinse your mouth with water prior to and during the breath test.

What is being tested?

Lactose tolerance tests measure hydrogen in the breath or changes in the level of glucose in the blood after a person is given a drink containing a standard amount of lactose, thus determining whether the individual is capable of proper digestion of lactose.

Lactose is a sugar with a complex structure (a disaccharide). It is found in milk and many other dairy products. Before it can be absorbed and used by the body, it must be broken down into two simpler sugars, glucose and galactose (monosaccharides). This digestion step is performed by lactase, an enzyme produced by cells lining the small intestine.

If an individual does not produce enough lactase (lactase deficient), then undigested lactose passes through the small intestine to the large intestine, where bacteria break it down, producing hydrogen gas and lactic acid. This process can cause the affected person to experience abdominal pain and bloating, flatulence (passing gas), and diarrhea within 30 minutes to 2 hours of consuming milk or other dairy products.

Almost all babies are born with the ability to digest lactose, but lactase production normally decreases as an individual ages. About 65-70% of the world’s population develops some degree of lactose intolerance by the time they reach adulthood. The intolerance can vary by race and ethnicity. While only about 5% of northern Europeans are lactose intolerant, more than 90% of Asians and Native Americans become lactose intolerant.

Two different types of lactose tolerance tests are available. In both of these, the person tested is given a liquid to drink that contains a standard amount of lactose. A sample for testing is taken immediately before, and a series of timed samples is collected at intervals after taking the lactose drink.

Hydrogen breath test
This is the test more commonly used to detect and diagnose lactose intolerance. This test measures hydrogen gas in breath samples taken before and after the lactose drink. With lactose intolerance, undigested lactose reaches the large intestine and is broken down by bacteria, producing excess hydrogen gas. The hydrogen gas enters the circulation and is eventually exhaled by the lungs and can be measured in the breath.

Glucose blood test
This is an alternate test sometimes used to detect and diagnose lactose intolerance. This test measures the glucose level in the blood samples taken immediately before and after the lactose drink. Since lactose is normally broken down to glucose and galactose, taking the lactose drink would normally lead to absorption of this glucose and result in an increase in blood glucose. In persons with lactose intolerance, there is inadequate breakdown of lactose and so this rise in blood glucose is not seen.

How is the sample collected for testing?

Breath samples are collected by blowing into a bag or other collection device. Blood samples are obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

Fasting is required before and during testing, and you must be off antibiotics and stomach medications, such as antacids, laxatives and stool softeners, for two weeks prior to the test. There are other medications that may interfere with the test, so consult your healthcare practitioner about the medications you are taking. You should also not exercise or smoke for several hours before testing. In some cases, additional instructions may be provided by the health practitioner and/or laboratory. For example, you may be asked to brush your teeth and then rinse your mouth with water prior to the hydrogen breath test and then again after drinking the liquid containing lactose. Follow any instructions you are given.

Common Questions

How is the used?

Lactose tolerance testing is used to help diagnose the cause of symptoms that suggest a reduced ability to digest lactose. These include abdominal pain and diarrhea following the ingestion of milk or dairy products. It may be ordered by itself or as part of a larger panel of tests when a secondary cause, such as malabsorption, is suspected. The hydrogen breath test is the more commonly ordered test. Doctors may also order this test as a part of a larger food sensitivity panel. The blood test is not ordered as frequently.

When is it ordered?

Testing is ordered when a person has signs and symptoms that suggest lactose intolerance that develop 30 minutes to 2 hours after ingesting milk or other dairy products. Some of these include:

  • Abdominal pain and bloating
  • Diarrhea
  • Nausea
  • Flatulence (passing gas)

What does the test result mean?

Hydrogen breath test
A baseline breath sample is taken before giving a lactose-loaded drink. If the hydrogen gas in a person’s breath significantly increases from the baseline, then it is likely that the person is lactose-intolerant.

If the breath samples are negative or low for hydrogen, then it is less likely that the person is lactose-intolerant. The signs and symptoms may be due to another cause. However, some people may have lactose intolerance even with a negative result. In these cases, the bacteria in the intestine may not produce hydrogen. This can be confirmed after ingestion of lactulose, a synthetic, non-digestible disaccharide sugar that is similarly broken down to hydrogen gas by intestinal bacteria. Since lactulose is not absorbed by the intestine, the ongoing lack of hydrogen gas production suggests a false negative indicating the person may still be lactose-intolerant.

Glucose blood test
Timed samples of blood are taken and measured for glucose. If the glucose levels do not increase and the person has symptoms consistent with lactose intolerance, then the condition is likely present. Increasing blood glucose levels over the course of the test indicates that signs and symptoms are unlikely due to lactose intolerance.

Care must be taken when interpreting results of the test. People who have diabetes may have an increase in blood glucose even when they do not produce enough lactase.

Is there anything else I should know?

Antibiotics taken within the last month or two prior to testing may decrease the number of normal bacteria in the large intestine and give a false-negative hydrogen breath test.

If food moves more quickly than usual through a person’s intestinal tract, that person may experience symptoms suggestive of lactose intolerance because the lactose has a shorter amount of time to be exposed to and broken down by lactase.

Bacterial overgrowth in the intestines (more bacteria present than normal) can cause symptoms similar to lactose intolerance, as can a variety of other gastrointestinal disorders.

Although it is not commonly done, it is possible to test for mutations in the gene that regulates lactase production (the LCT gene). Potentially, this could identify likely lactase deficiency and take the place of hydrogen breath testing.

Should everyone have lactose tolerance testing?

Many people have some degree of lactose intolerance, especially as they age, but general testing is not considered necessary.

Is the breath test used for other things?

Yes, other sugar “loads” can be given to examine other sugar (disaccharide) intolerances; however, lactose is the most commonly tested.

Other breath tests are available to help detect conditions, but they measure different substances and are unrelated to tolerance testing. Two examples are a breath test for Helicobacter pylori infection and alcohol (ethanol) testing.

Can I do anything to increase my production of lactase?

No, lactase concentrations do not change with lifestyle changes. However, many people with lactose intolerance are able to digest small amounts of milk and are often able to tolerate yogurt and hard cheeses.

Is there treatment available for lactose intolerance?

There is no specific treatment to cure lactose intolerance, but it can be managed. Lactose-reduced dairy products are available for those with the condition. Supplements that contain the lactase enzyme can be taken when dairy products are consumed to help break down the lactose and prevent signs and symptoms. Non-dairy sources of calcium and other nutrients found in dairy products are also available for those who cannot tolerate lactose.

Does lactose intolerance only apply to breast milk and cow's milk?

No, lactose is present in the milk of all mammals, including, for example, goat’s milk.

Does soy milk have lactose in it?

No. Most people with lactose intolerance can drink soy milk as well as other plant-based products, such as rice milk or almond milk.

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