Norm of d-Xylose Absorption Test (Xylose Tolerance Test)
Time After Ingestion
Serum D-Xylose Level
|Fasting||0 mg/dL||0 mmol/L|
|After ingesting 25 g|
|1-Hour sample||21–57 mg/dL||1.40–3.80 mmol/L|
|2-Hour sample||32–58 mg/dL||2.13–3.87 mmol/L|
|3-Hour sample||19–42 mg/dL||1.27–2.80 mmol/L|
|4-Hour sample||11–29 mg/dL||0.74–1.93 mmol/L|
|5-Hour sample||6–48 mg/dL||0.40–3.21 mmol/L|
|After ingesting 5 g|
|2-Hour sample||20–60 mg/dL||1.33–4.00 mmol/L|
|Children >10 years||D-Xylose level 1 hour after ingestion|
|Fasting||0 mg/dL||0 mmol/L|
|<6 months of age (after ingesting 0.5 g/kg)||15–58 mg/dL||1.00–3.87 mmol/L|
|6 months-16 years (after ingesting 0.5 g/kg)||20–58 mg/dL||1.33–3.87 mmol/L|
Grams of D-Xylose Excreted in Urine During 5 Hours After Ingestion
Fraction of Xylose Excreted in Urine
|Age 17–64 after ingesting 25 g||4–10 g||16%–40%|
|Age 17–64 after ingesting 5 g||1.2–2.0 g||20%–40%|
|≥65 after ingesting 25 g||3.5–10 g||14%–40%|
|≥65 after ingesting 5 g||1.2–2.0 g||20%–40%|
|Children||n/a, because dose varies by weight||16%–40% excreted in 5 hours|
Disaccharidase deficiencies, Hodgkin's disease, malabsorption, status post gastrectomy, radiation side effects of small intestine, and scleroderma.
Amyloidosis, ascariasis, blind loop syndrome, celiac disease, Crohn's disease, cystic fibrosis, diarrhea, immunoglobulin deficiency, massive bacterial overgrowth in small bowel, pellagra, postoperatively (after bowel resection), radiation enteritis, short bowel syndrome, tropical sprue, Whipple's disease, and any other jejunal mucosal disease.
Description of d-Xylose Absorption Test (Xylose Tolerance Test)
d-Xylose is a pentose (carbohydrate) that is not metabolized by the body and is normally absorbed by the proximal portion of the small bowel and excreted unchanged by the kidney into the urine. The test is used to distinguish malabsorption from maldigestion because it helps evaluate the efficiency of mucosal absorption efficiency. In clients with normal renal function, results indicate whether the absorptive abilities of the mucosa are impaired. In clients with malabsorption, both serum and urine values would be lower than the norms. Urine d-xylose is measured along with serum d-xylose to provide information related to renal retention. In clients with renal problems, the urine collection is not done.
Professional Considerations of d-Xylose Absorption Test (Xylose Tolerance Test)
Consent form NOT required.
- See Client and Family Teaching.
- Obtain a large brown urine container and three red-topped, marble-topped, or gold-topped tubes.
- Assess renal function laboratory data (BUN, creatinine).
- At 0800 (8 am), instruct the client to void and discard the sample.
- Draw a fasting blood sample of 4 mL and write on the tube the date and time collected and “fasting sample.”
- d-Xylose dose:
- a. Adults: Give 25 g of d-xylose dissolved in 250 mL of water by mouth.
- b. Children: Give 0.5 g of d-xylose per kilogram of body weight, up to 25 g.
- c. For clients unable to take 25 g, a 5-g dose may be used.
- Follow with 250 mL of water orally.
- No further fluids or food should be given until the test is completed.
- Collect all the urine voided for 5 hours after ingestion of d-xylose in a refrigerated container.
- Adults: Draw a 5-mL blood specimen for d-xylose levels 60 and 120 minutes after ingestion of d-xylose. Some tests may also include 3-hour, 4-hour, and 5-hour collections. Label the tube with the date and time collected as well as the number of hours since ingestion (e.g., “1 hour sample”).
- Children: Draw a 5-mL blood specimen for d-xylose levels 60 minutes after ingestion of d-xylose. Include the date and time collected and label as the 1-hour sample.
- Document on the laboratory requisition the total dose of d-xylose administered and the total volume of urine collected.
- Resume fluids and diet as prescribed.
Client and Family Teaching
- Adults must fast for 8 hours and children for 4 hours before drinking prescribed d-xylose.
- Do not eat foods containing pentoses: fruits, jams, jellies, and pastries.
- You will not be able to smoke during the test.
- You will need to rest quietly during the test. The d-xylose commonly causes mild diarrhea.
- The test involves specifically timed specimens.
- The test takes several hours. Bring reading material or other diversions to the test.
Factors That Affect Results
- Failure to collect all urine voided during the testing time will produce a falsely low result.
- Drugs that will increase absorption in the intestines include aspirin, atropine, and indomethacin. Other drugs that interfere with the test results include colchicine, digitalis, MAO inhibitors, nalidixic acid, neomycin, opium alkaloids, and phenelzine.
- Poor renal function will decrease urinary output, and vomiting will decrease the amount of d-xylose consumed or absorbed.
- The urine amount of d-xylose may be decreased by dehydration, delayed gastric emptying, renal insufficiency, reduced circulation, third spacing of fluid (such as in pregnancy and ascites), and hypothyroidism, but these will not affect the serum levels.
- Massive bacterial overgrowth in the small bowel may decrease the amount of d-xylose available for absorption by the small bowel and therefore decrease the serum and urine levels.
- Because an abnormal d-xylose test is suggestive of small bowel mucosal disease, a biopsy should be performed as the next step.