D-Xylose Absorption Test
The D-xylose absorption test measures the level of D-xylose, a type of sugar, in a blood or urine sample. This test is done to help diagnose problems that prevent the small intestine from absorbing nutrients in food.
D-xylose is normally easily absorbed by the intestines. When problems with absorption occur, D-xylose is not absorbed by the intestines, and its level in blood and urine is low.
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.
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 they drink the solution. For children, a blood sample may be taken 1 hour after they drink the solution. Another blood sample may be drawn 5 hours after the child drinks the solution.
You will need to collect all of the urine you produce for 5 hours after you drink the sugar solution. Sometimes urine is collected for 24 hours after you drink the sugar solution.
A health professional uses a needle to take a blood sample, usually from the arm.
- 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).
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is no pain while collecting a 5-hour urine sample.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
This test can cause dehydration. Make sure that you drink enough fluids to replace lost liquids after you have completed the test.
Drinking the D-xylose preparation may cause vomiting and diarrhea. Tell your doctor if you have problems after drinking the D-xylose solution.
Blood levels of D-xylose are highest about 2 hours after you drink the D-xylose solution. Almost all of the D-xylose is eliminated from the body in the urine within 5 hours. If the intestines can't absorb the D-xylose properly, the amount of D-xylose in the blood and urine will be very low.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
Low values may be caused by:
- A disease that interferes with the intestine's ability to absorb nutrients (malabsorption syndrome), such as celiac disease, Crohn's disease, or Whipple's disease.
- Inflammation of the lining of the intestine.
- Short bowel syndrome.
- An infection with a parasite, such as giardiasis or hookworm.
- An infection that causes vomiting (such as food poisoning or the flu).