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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.
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 .
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.
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.
- 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 4 L (1 gal). 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).
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.
There is no pain while collecting a 5-hour urine sample.
There is very little chance of a problem from having a blood sample taken from a vein.
- You may get a small bruise at the site. You can lower the risk of bruising by keeping pressure on the site for several minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
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.
The D-xylose test measures the level of D-xylose, a type of sugar, in a blood or urine sample.
Blood levels of D-xylose are highest about 2 hours after drinking 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.
Many conditions can change D-xylose levels. Your doctor will discuss any significant abnormal results with you in relation to your symptoms and past health.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Infants (5-gram dose):
Children (5-gram dose):
More than 20 mg/dL or more than 1.3 mmol/L
Adults (5-gram dose):
More than 20 mg/dL in 2 hours or more than 1.3 mmol/L
Adults (25-gram dose):
More than 25 mg/dL in 2 hours or more than 1.6 mmol/L
16%–33% of the D-xylose dose is found in the sample.
More than 16% of the D-xylose doseor more than 4 grams (g) is found in the sample.
Adults age 65 and older:
More than 14% of the D-xylose doseor more than 3.5 g is found in the sample.
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 foodborne illness or the flu).
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.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Current as of:
September 23, 2020
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Anne C. Poinier MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine
Jerome B. Simon MD, FRCPC, FACP - Gastroenterology
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