Also Known As
Xylose Tolerance Test
Formal Name
D-Xylose Absorption Test
This article was last reviewed on
This article waslast modified on January 16, 2019.
At a Glance
Why Get Tested?

To help evaluate your ability to absorb carbohydrates and help determine the cause of malabsorption

When To Get Tested?

When you have symptoms of malabsorption, such as persistent diarrhea and fatty stools

Sample Required?

Timed blood samples drawn from a vein in your arm and timed urine collection

Test Preparation Needed?

Fast for 8 hours and avoid foods high in pentose, such as jams, fruits, and pastries, for 24 hours prior to the test. Ask your healthcare provider if there are any changes to your medications that you should make.

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Xylose (d-xylose) is a simple sugar (carbohydrate) that is usually easily absorbed by the body. This test determines how well someone absorbs xylose. It measures the level of xylose in the blood and urine after a standard amount is ingested in order to evaluate the person's ability to absorb carbohydrates in general.

The body digests foods in three stages: first proteins, fats, and carbohydrates are broken down in the stomach by acids and enzymes and then in the small intestines by pancreatic enzymes and bile from the liver. They are then absorbed, primarily in the small intestines, and finally the nutrients are transported throughout the body and used or stored.

If there are not enough bile or pancreatic enzymes available, then carbohydrates and other foods cannot be properly digested. If a condition prevents the intestines from absorbing the nutrients, then they are "lost" by excretion in the stool. In both cases—improper digestion or absorption—the affected person can experience symptoms associated with malabsorption and, in severe cases, symptoms of malnutrition and vitamin deficiency.

This test is not routinely ordered and not widely available. Several major health organizations, including the American Gastroenterology Association, the World Gastroenterology Organization, and the British Society of Gastroenterology, do not include this test in their guidelines for evaluating possible malabsorption.

How is the sample collected for testing?

The xylose absorption test is a procedure that involves fasting prior to testing and emptying the bladder at the beginning of testing (this urine is not saved).

A fasting blood sample is obtained by inserting a needle into a vein in the arm and then the person is given a standard dose of xylose dissolved in water to drink. Typically, the dose is 25 grams of xylose, but if the person is unable to tolerate this amount, then a 5-gram dose may be used. For children, the dose is adjusted by weight. The person is then asked to rest quietly. Another blood sample is collected at 2 hours (1 hour for a child).

All urine is collected for 5 hours, starting from the time the dose is given.

The fasting blood, timed blood, and 5-hour urine samples are then tested for xylose.

Is any test preparation needed to ensure the quality of the sample?

Fast for 8 hours and avoid foods high in pentose, such as jams, fruits, and pastries, for 24 hours prior to the test. Ask your healthcare provider if there are any necessary medication changes.

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Common Questions
  • How is it used?

    The xylose absorption test may be ordered to help determine whether a person is absorbing carbohydrates normally and to distinguish between malabsorption disorders caused by insufficient pancreatic enzymes or bile and those due to dysfunction of the intestines.

    (For more on xylose (d-xylose) and how the test is done, see the "What is being tested?" section.)

    It may be ordered as a follow-up test if other test results, such as fecal fat, suggest malabsorption but do not reveal its cause.

    This is not a routine test. Its use and availability has declined over time. Major health organizations, including the American Gastroenterology Association, the World Gastroenterology Organization, and the British Society of Gastroenterology, do not include this test in their guidelines for evaluating possible malabsorption.

  • When is it ordered?

    This test may be performed when a person has signs and symptoms of malabsorption such as:

    • Fatty stools that are loose and foul-smelling (steatorrhea)
    • Persistent diarrhea
    • Abdominal pain, cramps, bloating, and gas
    • Weight loss
    • Failure to thrive (in children)

    Testing may be ordered when a healthcare practitioner wants to determine whether a patient's intestines are absorbing carbohydrates normally and/or wants to distinguish between different possible causes of malabsorption.

  • What does the test result mean?

    With the xylose absorption testing procedure, high blood and urine levels of xylose are normal. They indicate good xylose absorption by the intestines. This suggests that the tested person's symptoms are likely due to another cause, such as pancreatic insufficiency or bile insufficiency.

    High blood levels but low urine levels may be seen in someone with kidney dysfunction. In this case, it is the blood levels that will be used to evaluate the individual for malabsorption.

    Low levels of blood and urine xylose indicate poor absorption. A variety of conditions that affect digestion and absorption may cause decreased xylose levels. These may include bacterial overgrowth in the intestines, parasitic infections, a shortened bowel (such as from surgery) and celiac disease.

    Depending upon a person's clinical situation, an abnormal xylose absorption test may be followed by additional blood or stool tests to try to isolate the cause and/or by a small-bowel biopsy to look at intestinal cells.

  • Is there anything else I should know?

    Several drugs can affect test results, including aspirin, digitalis, MAO inhibitors, metformin, nalidixic acid, opium alkaloids, atropine, and indomethacin.

    Dehydration, exercise, and the rate that the stomach empties may affect test results.

    Some people may experience nausea, vomiting, or diarrhea from the xylose dose.

  • Can this test be performed in my healthcare practitioner's office?

    The samples may be collected in a healthcare practitioner's office, but more often the test is conducted at a sample drawing (phlebotomy) station. Sample analysis must be done in a laboratory and may need to be sent to a reference laboratory.

  • Is it really necessary to save all of my urine over 5 hours?
    Yes. In order to accurately interpret absorption and excretion during this time period, you should save all of your urine.
  • Why is the fasting blood sample collected?
    This measurement of xylose in a fasting sample is used as a baseline. It should be negative or very low prior to receiving the xylose dose.
  • Is there a reason to repeat a xylose test?

    Usually only if the healthcare provider feels that the first set of test results was compromised in some way, such as due to an interfering medication or because the person tested was unable to tolerate and retain the normal xylose dose (nausea and vomiting).

  • Why would malabsorption cause vitamin deficiency?

    Malabsorption can cause vitamin deficiency because the vitamins cannot be absorbed normally and because fat-soluble vitamins such as A, E, K, and D can be "trapped" in and eliminated with fat in fatty stools.

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Related Content

On This Site

Tests: Fecal Fat, Celiac Disease Tests

Conditions: Malabsorption, Diarrhea, Cystic Fibrosis, Celiac Disease, Pancreatic Insufficiency, Malnutrition

Elsewhere On The Web

MedlinePlus Medical Encyclopedia: Malabsorption
American Gastroenterological Association: Patient Center
National Institute of Diabetes and Digestive and Kidney Diseases: Your Digestive System and How it Works
American Academy of Pediatrics: Malabsorption

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View Sources

Sources Used in Current Review

Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2011, Pp 322-323.

(December 16, 2014) Goebel S. Malabsorption. Medscape Reference. Available online at http://emedicine.medscape.com/article/180785-workup#c5. Accessed May 2016.

Haldeman-Englert C. (Updated 2014 March 14). D-xylose absorption. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003606.htm. Accessed May 2016.

(© 1995-2016). Xylose Absorption Test (Adult - 25g dose). Mayo Clinic Mayo Medical Laboratory [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/57321. Accessed May 2016.

(© 2016). Xylose Absorption Test (Adult - 5g dose): 0020615. ARUP's Laboratory Test Directory [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0020615.jsp. Accessed May 2016.

Lehrer J. (Updated 2014 August 11). Malabsorption. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000299.htm. Accessed May 2016.

Sources Used in Previous Reviews

Dugdale, D. (Updated 2010 January 20). D-xylose absorption. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003606.htm. Accessed June 2011.

(© 1995-2011). Unit Code 91713: Xylose Absorption Test (Adult - 25g dose). Mayo Clinic Mayo Medical Laboratory [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/91713. Accessed June 2011.

(© 2006-2011). Xylose Absorption Test (Adult - 5g dose): 0020615. ARUP's Laboratory Test Directory [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0020615.jsp. Accessed June 2011.

Dugdale, D. (Updated 2010 July 7). Malabsorption. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000299.htm. Accessed June 2011.

Delgado, J. and Grenache, D. (Updated 2010 November). Malabsorption. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/Malabsorption.html?client_ID=LTD. Accessed June 2011.

Ruiz, A. (Revised 2008 January). Malabsorption. Merck Manual for Healthcare Professionals [On-line information]. Available online through http://www.merckmanuals.com. Accessed June 2011.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 1054-1055.

Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 304-305.

Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 1148-1151.

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