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:
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.
A normal result depends on how much D-xylose is given. Usually the test results are either positive or negative. A positive result means that D-xylose is found in the blood or urine and is therefore being absorbed by the intestines.
Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your doctor about the meaning of your specific test results.
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:
Fainting or feeling light-headed
Hematoma (blood accumulating under the skin)
Infection (a slight risk any time the skin is broken)
Multiple tests may be necessary to determine the reason for malabsorption.
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.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and George F. Longstreth, MD, Department of Gastroenterology, Kaiser Permanente Medical Care Program, San Diego, California. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.