D-xylose absorption

Alternative names
Xylose tolerance test

This test measures the intestines’ ability to absorb D-xylose, a simple sugar, as an indicator of whether nutrients are being properly absorbed.

How the test is performed

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. An initial blood and urine sample are taken

You are given 25 grams of D-xylose in 8 ounces of water. Blood is collected after 2 hours, and sometimes again after 5 hours. Urine is re-tested after 8 hours. Urine excretion over a 5-hour period is also determined (all of the urine during 5 hours is collected).

Urine collection:
Collect a “clean-catch” (“midstream”) urine sample. To obtain a clean-catch sample, men or boys should wipe clean the head of the penis. Women or girls need to wash the area between the lips of the vagina with soapy water and rinse well. As you start to urinate, allow a small amount to fall into the toilet bowl (this clears the urethra of contaminants). Then, in a clean container, catch about 1 to 2 ounces of urine and remove the container from the urine stream. Give the container to the health care provider or assistant.

Thoroughly wash the area around the urethra. Open a Urine collection bag (a plastic bag with an adhesive paper on one end), and place it on your infant. For males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, the bag is placed over the labia. Place a diaper over the infant (bag and all).

Check your baby frequently and remove the bag after the infant has urinated into it. For active infants, this procedure may take a couple of attempts-lively infants can displace the bag, causing an inability to obtain the specimen. The urine is drained into a container for transport back to the health care provider.

Blood collection:
Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
Fast for 8 to 12 hours before the test. Consult the health care provider about the need to withhold drugs that can affect test results (see “special considerations”).

If the collection is being taken from an infant, a couple of extra collection bags may be necessary.

For infants and children:
The preparation you can provide for this test depends on your child’s age and previous experience. For specific information regarding how you can prepare your child, see the following topics:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel

When the needle is inserted to collect the blood, a stinging sensation or prick will be felt. After the blood has been drawn, the puncture site may throb for a few minutes, and may feel bruised for a day or two.

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

Why the test is performed
This test may be performed to help evaluate cases of:

  • persistent diarrhea  
  • unexplained weight loss  
  • suspected malnutrition  
  • general weakness

This test is especially useful in distinguishing nutrient absorption problems that are due to disease of the intestines from those due to disease of the pancreas.

Normal Values
The value varies depending 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 and/or urine and is therefore being absorbed by the intestines.

blood: greater than 25 mg/dl (milligrams per deciliter) at 2 hours after ingestion
urine: greater than 16% of ingested dose excreted within 5 hours of ingestion

What abnormal results mean
Lower than normal values may be seen in:

  • Crohn’s disease  
  • Giardia lamblia infestation  
  • hookworm infestation  
  • lymphatic obstruction  
  • radiation enteropathy  
  • small intestinal bacterial overgrowth  
  • celiac disease (sprue)  
  • viral gastroenteritis  
  • Whipple’s disease

What the risks are

The risks associated with having blood drawn are:

  • excessive bleeding  
  • fainting or feeling lightheaded  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Special considerations
Multiple tests may be necessary to determine the reason for malabsorption (inadequate absorption of nutrients from the intestinal tract).

A failure to restrict activity may change test results.

Drugs that can affect test results include aspirin, atropine, indomethacin, isocarboxazid, and phenelzine.

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.

Johns Hopkins patient information

Last revised: December 6, 2012
by Simon D. Mitin, M.D.

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