Xylose Absorption Test

The xylose absorption test may be ordered to help determine whether a person is absorbing carbohydrates normally, and to distinguish between digestive disorders caused by insufficient pancreatic enzymes or bile and those due to dysfunction of the intestines.

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.

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.

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.

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.

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.

Yes. In order to accurately interpret absorption and excretion during this time period, you should save all of your urine.

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.

Usually only if the health care 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).

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.