What Is a D-Xylose Absorption Test?

A D-xylose absorption test is used to check how well your intestines are absorbing a simple sugar called D-xylose. From the results of the test, your doctor can infer how well your body is absorbing nutrients.

D-xylose is a simple sugar that occurs naturally in many plant foods. Your intestines usually absorb it easily, along with other nutrients. To see how well your body is absorbing D-xylose, your doctor will usually first use blood and urine tests. These tests will show low D-xylose levels in your blood and urine if your body is not absorbing D-xylose well.

The D-xylose absorption test is not commonly done. However, one instance when your doctor may prescribe this test is when earlier blood and urine tests show that your intestines are not absorbing D-xylose properly. In this case, your doctor may want you to carry out the D-xylose absorption test to determine if you have malabsorption syndrome. This is caused when your small intestine, which is responsible for most of your food digestion, can’t absorb sufficient nutrients from your daily diet. Malabsorption syndrome can cause symptoms such as weight loss, chronic diarrhea, and extreme weakness and fatigue.

You should not eat foods containing pentose for 24 hours before a D-xylose absorption test. Pentose is a sugar that is similar to D-xylose. Foods high in pentose include:

  • pastries
  • jellies
  • jams
  • fruits

Your doctor may advise you to stop taking medicines such as indomethacin and aspirin prior to your test, as these can interfere with the results.

You should not eat or drink anything except water for eight to 12 hours prior to the test. Children should avoid eating and drinking anything but water for four hours prior to the test.

The test requires both a blood and urine sample. Your healthcare provider will ask you to drink 8 ounces of water containing 25 grams of D-xylose sugar. Two hours later, they’ll collect a blood sample. You’ll need to give another blood sample after another three hours. After eight hours, you’ll need to give a urine sample. The amount of urine you produce over a five-hour period will also be measured.

The Blood Sample

Blood will be drawn from a vein in your lower arm or the back of your hand. First your healthcare provider will swab the site with antiseptic, and will then wrap an elastic band around the top of your arm to cause the vein to swell with blood. Your healthcare provider will then insert a fine needle into the vein and collect a blood sample into a tube attached to the needle. The band is removed and gauze is applied to the site to prevent any further bleeding.

The Urine Sample

You’ll begin collecting your urine the morning on the day of the test. Don’t bother collecting the urine from when you first get up and empty your bladder. Start collecting urine from the second time you urinate. Make a note of the time of your second urination so your doctor knows when you began your five-hour collection. Collect all your urine over the next five hours. Your healthcare provider will provide you with a large, sterile container that usually holds about 1 gallon. It’s easiest if you urinate into a small container and add the sample to the larger container. Be careful not to touch the inside of the container with your fingers. Don’t get any pubic hair, stool, menstrual blood, or toilet paper in the urine sample. These might contaminate the sample and skew your results.

Your test results go to a laboratory for analysis. If your tests show you have abnormally low levels of D-xylose, it could mean you have one of following conditions:

  • short bowel syndrome, a disorder that may occur in people who have had at least one-third of their bowel removed
  • infection by a parasite such as hookworm or Giardia
  • inflammation of the intestinal lining
  • food poisoning or the flu

As with any blood test, there’s minimal risk of minor bruising at the needle site. In rare cases, the vein may become swollen after blood is drawn. This condition, known as phlebitis, can be treated with a warm compress several times each day. Ongoing bleeding could be a problem if you suffer from a bleeding disorder or if you’re taking blood-thinning medication such as warfarin (Coumadin) or aspirin.

If your doctor suspects you have malabsorption syndrome, they may recommend a test to examine the lining of your small intestine.

If you have an intestinal parasite, your doctor will do an additional test to see what the parasite is and how to treat it.

If your doctor believes you have short bowel syndrome, they’ll recommend dietary changes or prescribe medication.

Depending on the results of your test, your doctor will work with you to create an appropriate treatment plan.