Xylose Testing

What is a xylose test?

Xylose, also known as D-xylose, is a type of sugar that is normally easily absorbed by the intestines. A xylose test checks the level of xylose in both blood and urine. Levels that are lower than normal can mean there is a problem with your body's ability to absorb nutrients.

Other names: xylose tolerance test, xylose absorption test, D-xylose tolerance test, D-xylose absorption test

What is it used for?

A xylose test is most often used to:

• Help diagnose malabsorption disorders, conditions that affect your ability to digest and absorb nutrients from food
• Find out why a child is not gaining weight, especially if the child seems to be eating enough food

Why do I need a xylose test?

You may need this test if you have symptoms of a malabsorption disorder, which include:

• Persistent diarrhea
• Abdominal pain
• Bloating
• Gas
• Unexplained weight loss, or in children, the inability to gain weight

What happens during a xylose test?

A xylose test involves getting samples from both blood and urine. You will be tested before and after you drink a solution that contains 8 ounces of water that is mixed with a small amount of xylose.

For the blood tests:

• A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial.
• Next, you will drink the xylose solution.
• You will be asked to rest quietly.
• Your provider will give you another blood test two hours later. For children, it may be one hour later.

For the urine tests, you will need to collect all the urine you produce for five hours after you have taken the xylose solution. Your health care provider will give you instructions on how to collect your urine during the five-hour period.

Will I need to do anything to prepare for the test?

You will need to fast (not eat or drink) for eight hours before the test. Children younger than 9 years old should fast for four hours before the test.

For 24 hours before the test, you will need to not eat foods high in a type of sugar known as pentose, which is similar to xylose. These foods include jams, pastries, and fruits. Your provider will let you know if you need to take any other preparations.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

The xylose solution may make you feel nauseous.

There is no risk to having a urine test.

What do the results mean?

If your results showed lower than normal amounts of xylose in the blood or urine, it may mean you have a malabsorption disorder, such as:

• Celiac disease, an autoimmune disorder that causes a serious allergic reaction to gluten. Gluten is a protein found in wheat, barley, and rye.
• Crohn's disease, a condition that causes swelling, inflammation, and sores in the digestive tract
• Whipple disease, a rare condition that prevents the small intestine from absorbing nutrients

Low results may also be caused by an infection from a parasite, such as:

• Hookworm
• Giardiasis

If your xylose blood levels were normal, but urine levels were low, it may be a sign of kidney disease and/or malabsorption. You may need more tests before your provider can make a diagnosis.

If you have questions about your results or your child's results, talk to your health care provider.

Is there anything else I need to know about xylose testing?

A xylose test takes a long time. You may want to bring a book, game, or other activity to keep yourself or your child occupied while you wait.

Learn more about laboratory tests, reference ranges, and understanding results.