What Is a D-xylose Absorption Test?
A D-xylose absorption test is performed in order to determine how adequately your intestines are absorbing D-xylose and by inference how well you absorb carbohydrates.
D-xylose is a simple sugar that occurs naturally in many plant foods. The body does not use it or change D-xylose into something usable. It is normally absorbed easily, along with other nutrients, by the intestines. When problems with absorption do occur, your doctor will be able to see that D-xylose levels in your body are low by taking blood and urine tests. The D-xylose absorption test is not a common test.
What the Test Addresses
If your intestines are not absorbing D-xylose properly, your doctor may want to administer this test to determine if you are suffering from a malabsorption syndrome. This is caused when your small intestine—which is responsible for most of your food digestion—is unable to absorb sufficient nutrients from your daily diet. Malabsorption syndrome can cause symptoms such as weight loss, chronic diarrhea, and extreme weakness and fatigue.
Preparation for the Test
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, and 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 blood draw. Children should avoid eating and drinking anything but water for four hours prior to the test.
Where and How Is the Test Administered
Your doctor will perform the test in his or her office. Afterward, the results are sent to a laboratory to be analyzed.
The test requires both a blood test and a urine sample test. Your doctor will ask you to drink 8 ounces of water containing 25 grams of D-xylose sugar. Two hours later, he or she will collect a blood sample. You will need to give another blood sample after another three hours. After eight hours you will 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 the inside of your lower arm or the back of your hand. First the doctor will swab the site with antiseptic, and may then wrap an elastic band around the top of your arm to cause the vein to swell with blood. Your doctor 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 applied to the site to prevent any further bleeding.
The Urine Sample
You will begin collecting your urine in the morning on the day of your test. Do not bother collecting the urine from when you first get up and empty your bladder. Start collecting with the second time you urinate. Make a note of the time of your second urination so the doctor will know when you began your five-hour collection. Collect all your urine over the next five hours. Your doctor will provide you with a large, sterile container that usually holds about 1 gallon. It is 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. Do not get any pubic hair, stool, menstrual blood, or toilet paper in the urine sample. These might contaminate the sample and skew your results.
Understanding the Results
If your tests show you have abnormally low levels of D-xylose, this could mean you are suffering from one of the 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
Low levels of C-xylose can indicate a malabsorption syndrome, such as:
- celiac disease—a disease that causes damage to the intestinal lining and prevents it from absorbing nutrients
- Whipple’s disease—a rare condition that prevents nutrient absorption, caused by Tropheryma whippelii bacteria
What Are the Risks of the Test?
As with any blood test, there are minimal risks of experiencing 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 are taking blood-thinning medication such as warfarin (Coumadin) or aspirin.
Following up After a D-Xylose Absorption Test
If your doctor suspects you are suffering from a malabsorption syndrome he or she may recommend a test to examine the lining of your small intestine.
If you are suffering from an intestinal parasite, the doctor will administer an additional test to determine what the parasite is and how to treat it.
If your doctor believes that you are suffering from short bowel syndrome, he or she may recommend dietary changes or prescribe medication.