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D-xylose absorption

Definition

D-xylose absorption is a laboratory test to determine how well the intestines absorb a simple sugar (D-xylose). The test helps determine if nutrients are being properly absorbed.

Alternative Names

Xylose tolerance test

How the test is performed

The test requires a blood and urine sample. For information on how these samples are obtained, see:

• Clean catch urine specimen
• Venipuncture (blood draw)

There are several ways to perform this test. A typical procedure is described below, but make sure you follow the specific instructions you are given.

You will be asked to drink 8 ounces of water that contains 25 grams of a sugar called D-xylose. Two hours later, you will have a blood sample collected. You may also need to give blood again after 5 hours. Your urine sample is tested 8 hours later. The amount of urine you produce over a 5-hour period is also determined. Your health care provider will tell you how to collect all of the urine during a 5-hour period.

How to prepare for the test

Do not eat or drink anything (even water) for 8 to 12 hours before the test. Do not exercise before the test. A failure to restrict activity may affect test results.

Your health care provider may tell you to stop taking certain drugs that can affect test results. Drugs that can affect test results include aspirin, atropine, indomethacin, isocarboxazid, and phenelzine. Never stop taking any medicine without first talking to your doctor.

How the test will feel

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

Urine is collected as part of normal urination with no discomfort.

Why the test is performed

Your doctor may order this test if you have:

• General weakness
• Persistent diarrhea
• Unexplained weight loss
• Signs of malnutrition

This test is especially useful in determining if nutrient absorption problems are due to a disease of the intestines or a disease of the pancreas.

Normal Values

Normal values depend on the amount of D-xylose administered. Usually the test is registered as either positive or negative. Positive indicates that D-xylose is found in the blood or urine and is therefore being absorbed by the intestines.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

What abnormal results mean

Lower than normal values may be seen in:

• Celiac disease (sprue)
• Crohn's disease
• Giardia lamblia infestation
• Hookworm infestation
• Lymphatic obstruction
• Radiation enteropathy
• Small intestinal bacterial overgrowth
• Viral gastroenteritis
• Whipple's disease

What the risks are

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Special considerations

Multiple tests may be necessary to determine the reason for malabsorption.

References

Hogenauer C, Hammer HF. Maldigestion and malabsorption. In: Feldman M, Friedman LS, Sleisenger MH, eds. Sleisenger & Fordtran's Gastrointestinal and Liver Disease. 8th ed. Philadelphia, Pa: WB Saunders; 2006:chap 98.

Semrad CE, Powell DW. Approach to the patient with diarrhea and malabsorption. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 143.