Lactose Tolerance Tests-Why am I having this test?

Lactose tolerance tests may be done to help diagnose:

• Lactose intolerance.
• Problems with digestion or absorption in the intestines.
• Imbalance of bacteria in the small intestine.

The tests may be done to evaluate:

• Adults who have frequent diarrhea or vomiting, especially after eating or drinking dairy products.
• Infants who are failing to thrive.

What is being tested?

Lactose tolerance tests measure how well your body is able to break down (digest) or tolerate lactose. Lactose is a type of sugar that is found in dairy products. Digesting it requires an enzyme called lactase that is normally found in the small intestine. When lactose is digested properly, it is broken down into the simple sugars called glucose and galactose. These enter the bloodstream and are used throughout the body for energy.

For the lactose tolerance tests, you will drink a lactose-rich mixture. After you drink the mixture, measurements will be done on a sample of your blood or breath.

• If the lactose was digested properly, it will cause an increase in the glucose levels in your blood.
• If the lactose was not digested properly, it will be broken down into hydrogen and lactic acid, and this will cause an increase in hydrogen levels in your breath.

What kind of sample is taken?

Blood or breath samples are required for this test. Multiple samples of blood or breath will be taken over a 2-hour period to see how drinking the lactose mixture affects the measurements.

For the blood test, samples are usually collected by inserting a needle into a blood vessel.

For the breath test, samples are usually collected by having you blow into a bag.

How do I prepare for this test?

• Do not eat or drink anything for 12 hours before the test, or as told by your health care provider.
• Avoid strenuous exercise for at least 8 hours before the test, or as told by your health care provider.
• If you smoke, do not smoke for at least 8 hours before the test if you are going to have a breath test.

Tell a health care provider about:

• All medicines you are taking, including vitamins, herbs, eye drops, creams, and over-the-counter medicines.
• Any medical conditions you have, including diabetes.

How are the results reported?

Your test results will be reported as values. Your health care provider will compare your results to normal ranges that were established after testing a large group of people (reference ranges). Reference ranges may vary among labs and hospitals. For this test, common reference ranges are:

• Blood glucose levels, adult: Rise greater than 20 mg/dL (1.1–1.7 mmol/L).
• Breath test: Less than 50 ppm hydrogen above baseline.
• No abdominal cramps or diarrhea after drinking lactose.

What do the results mean?

Abnormally low levels of blood glucose and abnormally high levels of exhaled hydrogen may be related to:

• Lactose intolerance.
• Malabsorption diseases of the intestine, such as inflammatory bowel disease or short gut syndrome.

Talk with your health care provider about what your results mean. Several factors and conditions can affect the results of lactose tolerance tests. Your health care provider will rule them out in order to make an accurate diagnosis.

Questions to ask your health care provider

Ask your health care provider, or the department that is doing the test:

• When will my results be ready?
• How will I get my results?
• What are my treatment options?
• What other tests do I need?
• What are my next steps?

Summary

• Lactose tolerance tests measure how well your body is able to break down (digest) or tolerate lactose. Lactose is a type of sugar that is found in dairy products.
• Lactose tolerance testing may be done with a blood test or a breath test. Testing is done after you drink a lactose-rich mixture. Multiple samples of blood or breath are taken over 2 hours.
• Talk with your health care provider about what your test results mean.