D-Xylose Absorption Test

Why It Is Done

A test for D-xylose is done to:

• Check to see if malabsorption syndrome is causing symptoms, such as chronic diarrhea, weight loss, and weakness. A person with malabsorption syndrome is unable to absorb nutrients, vitamins, and minerals from the intestinal tract into the bloodstream.
• Find the cause of a child's failure to gain weight, especially when the child seems to be eating enough food.

How To Prepare

For 24 hours before a D-xylose test, do not eat foods high in pentose, a sugar similar to D-xylose. These foods include fruits, jams, jellies, and pastries.

Medicines such as aspirin and indomethacin can interfere with the results of a D-xylose test. For this reason, your doctor may instruct you to temporarily stop these medicines before the test.

Do not eat or drink anything except water for 8 to 12 hours before having this test. Children younger than 9 years old should not eat or drink anything except water for 4 hours before the test.

A D-xylose test can take a long time. It might be a good idea to bring something you can do quietly while you wait, such as a book to read.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form (What is a PDF document?).

How It Is Done

The amount of D-xylose in urine and blood samples is measured before and after you drink a D-xylose solution. To begin the test, a sample of your first urine of the day and a sample of your blood are collected.

Next you will drink a D-xylose solution. For adults, a blood sample is usually taken 2 hours after drinking the solution. For children, a blood sample may be taken 1 hour after drinking the solution. Another blood sample may be drawn 5 hours after drinking the solution.

You will need to collect all of the urine you produce for 5 hours after drinking the sugar solution. Sometimes urine is collected for 24 hours after drinking the sugar solution.

Blood test

The health professional taking a sample of your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

Urine test

• You start collecting your urine in the morning. When you first get up, empty your bladder, but do not save this urine. Write down the time that you urinated to mark the beginning of your 5-hour collection period.
• For the next 5 hours, collect all your urine. Your doctor or lab will usually provide you with a large container that holds about 1 gal (4 L). The container has a small amount of preservative in it. Urinate into a small, clean container and then pour the urine into the large container. Do not touch the inside of the container with your fingers.
• Keep the large container in the refrigerator during the collection period.
• Empty your bladder for the final time at or just before the end of the 5-hour period. Add this urine to the large container and record the time.
• Do not get toilet paper, pubic hair, stool (feces), menstrual blood, or other foreign matter in the urine sample.

You will not be allowed to eat until the test is completed.

How It Feels

Drinking the D-xylose solution can make you feel sick to your stomach (nauseated).

Blood test

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Urine test

There is no pain while collecting a 5-hour urine sample.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Vomiting after taking the D-xylose solution.
• Being dehydrated before starting the test.
• Eating foods high in pentose within 24 hours before the test. These foods include fruits, jams, jellies, and pastries.
• Large amounts of bacteria in the intestines.
• Kidney disease or bladder problems that prevent you from completely emptying your bladder.
• Medicines, such as antibiotics, aspirin, and heart medicines. Many medicines can affect D-xylose test results.
• Physical activity during the test. You will be instructed to rest quietly until the test is complete.
• Conditions that cause food to remain in the stomach for a long time.

What To Think About

• If you have an abnormally high amount of bacteria in your intestines, you may have to take antibiotics for a day or two before the test.
• This test can cause dehydration. Make sure that you drink enough fluids to replace lost liquids after you have completed the test. Tell your doctor if you have problems with diarrhea after drinking the D-xylose solution.
• Blood D-xylose levels are generally considered more reliable than urine levels in children younger than 12 years old.
• A test that looks at the lining of the small intestines (upper gastrointestinal series) may be used if Crohn's disease or another malabsorption syndrome is suspected. To learn more, see the topic Upper Gastrointestinal Series.