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Xylose Absorption Test

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Also known as: Xylose Tolerance Test
Formal name: D-Xylose Absorption Test

The Test Sample

What is being tested?

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 is 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 doctor if there are any necessary medication changes.