At a Glance
Why Get Tested?
When to Get Tested?
When you have symptoms such as abdominal pain, bloating, gas, and diarrhea after consuming milk and other dairy products
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. 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.
The Test Sample
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, 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, with about 90-95% of northern Europeans retaining their ability to digest lactose and 95-100% of Asians and Native Americans becoming lactose intolerant.
Two different types of lactose tolerance tests are available. They both involve the collection of a fasting sample. The person tested is given a liquid to drink that contains a standard amount of lactose. A series of timed samples is then collected and tested.
Hydrogen breath test
This is the primary test used to detect and diagnose lactose intolerance. This test measures hydrogen gas in breath samples. 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.
Lactose tolerance blood test
This is a secondary test sometimes used to help diagnose lactose intolerance. This test measures the glucose level in the blood samples. It detects the conversion of the ingested lactose into glucose and galactose.
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. Rarely, a stool sample may be collected in a clean container for analysis.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Fasting is required before and during testing. The person being tested should not exercise or smoke for several hours before testing. In some cases, additional instructions may be provided by the doctor 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.
Ask a Laboratory Scientist
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
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