Xylose Testing
Reviewed
Also called: xylose tolerance test, xylose absorption test, D-xylose tolerance test, D-xylose absorption test
A xylose test checks the level of xylose in both your blood and urine to see how well you absorb carbohydrates.
Specimen: Blood (serum or plasma) and urine (timed 5-hour collection)
Why this test may be ordered
- To help diagnose malabsorption disorders.
- To find out why a child is not gaining weight despite adequate food intake.
Preparation and collection
You will need to fast (not eat or drink) for eight hours before the test. Children younger than nine years old should fast for four hours before the test. For 24 hours before the test, you must avoid foods high in a type of sugar known as pentose, which is similar to xylose. You may need to stop taking certain medicines before this test, so tell your provider about everything you take. But don't stop taking any medicines unless your provider tells you to.
A xylose test involves taking timed samples of your blood and urine. You will be tested before and after drinking a solution containing eight ounces of water mixed with a small amount of xylose. Your provider will take another blood sample two hours later. After you drink the xylose solution, you will collect all your urine for the next five hours.
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly. The xylose solution may make you feel nauseous. There is no risk to having a urine test.
Understanding results
Lower-than-normal xylose levels can mean there is a problem with your body's ability to absorb nutrients.
- High (normal) xylose levels: High levels of xylose are normal. It means that your intestines are absorbing the xylose. If your results are high, that may mean that your symptoms are not caused by malabsorption. Limitation: Xylose testing is no longer routinely ordered to check for malabsorption, and it may not be available everywhere. Next step: Talk to your provider if you have questions about your or your child's results.
- Low xylose levels in blood or urine: If your results showed lower-than-normal levels of xylose in the blood or urine, it may mean you have a malabsorption disorder, such as: Celiac disease, a chronic (long-lasting) digestive and immune disorder that damages your small intestine. Crohn's disease, a chronic condition that causes inflammation in your digestive tract. Limitation: Lower-than-normal levels may also be caused by an infection from a parasite, such as: Hookworm, Giardiasis. Next step: You may need more tests before your provider can make a diagnosis.
- Normal blood xylose with low urine xylose: If your xylose blood levels were normal but urine levels were low, it may be a sign of kidney disease and/or malabsorption. Limitation: To understand the results of your xylose testing, your provider may consider your symptoms, medical history, and the results of other tests. Next step: You may need more tests before your provider can make a diagnosis.
- The xylose solution may cause nausea.
- False-positive or false-negative results are possible; your provider will interpret results alongside symptoms, history, and other tests.
- This test is no longer routinely ordered and may not be available everywhere.
What to do next
To understand the results of your xylose testing, your provider may consider your symptoms, medical history, and the results of other tests. Talk to your provider if you have questions about your or your child's results.
- Do you have chronic diarrhea, abdominal pain, bloating, gas, or unexplained weight loss?
- Is your child not gaining weight even though they seem to eat enough?
- Have other tests (e.g., fecal fat test) suggested a need for xylose testing?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.