Xylose Testing
Reviewed
Also called: xylose tolerance test, xylose absorption test, D-xylose tolerance test, D-xylose absorption test
Xylose testing measures xylose levels in blood and urine after drinking a xylose solution to assess how well your intestines absorb carbohydrates. It is used to help diagnose malabsorption disorders or evaluate why a child is not gaining weight. Results are interpreted by your healthcare professional.
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
Fast for 8 hours (or 4 hours for children under 9 years). For 24 hours before the test, avoid foods high in pentose sugar (e.g., jams, pastries, fruits). Tell your provider about all medicines you take; you may need to stop some before the test. Follow any additional instructions from your provider.
Blood draw (venous puncture; may have slight pain or bruising that goes away quickly). Urine collection (no known risk according to MedlinePlus).
According to MedlinePlus: 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
The test checks how well your intestines absorb xylose (a carbohydrate). Results are interpreted by your healthcare professional and may require follow-up testing.
- High xylose levels in blood and/or urine: Your intestines are absorbing xylose normally. High levels are normal and suggest your symptoms are not caused by malabsorption. Limitation: High levels do not rule out other causes of symptoms; further evaluation may still be needed. Next step: routine
- Low xylose levels in blood and/or urine: May indicate a malabsorption disorder (e.g., celiac disease, Crohn's disease, Whipple disease) or a parasitic infection (e.g., hookworm, giardiasis). Limitation: Low levels are not specific for a single condition; additional testing is needed before a diagnosis can be made. Next step: prompt
- Normal blood xylose with low urine xylose: Normal blood xylose but low urine xylose could be a sign of kidney disease and/or malabsorption. Limitation: This pattern requires further evaluation to distinguish between kidney disease and malabsorption. Next step: prompt
- 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
Your healthcare professional will consider your symptoms, medical history, and the results of other tests to make a diagnosis. More tests may be needed before a diagnosis can be made.
- 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?