Patient Test Information

Xylose Absorption Test

Also known as:

Xylose Tolerance Test

Formal name:

D-Xylose Absorption Test

Related tests:

Fecal Fat, Celiac Disease Tests

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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.

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.

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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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.

  1. 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.

  2. 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.

  3. 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.

  4. 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).

  5. 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.