The D-xylose absorption or tolerance test is used for evaluating patients with possible malabsorption, especially if celiac disease (gluten sensitive enteropathy) is suspected. D-xylose is a pentose sugar with a molecular weight of 150.15 grams. It is passively absorbed from the small intestine (particularly the jejunum), with 30-40% of the absorbed amount excreted in the urine. Failure to excrete a certain amount of D-xylose in the urine is seen in enterogenous malabsorption; pancreatic enzymes are not required for D-xylose absorption. Patients with renal disease may show reduced urine excretion, and should also have plasma samples drawn at specified times during the test.
Patient Preparation:
(1) Fasting: Older children and adults are fasted at least 8 hours. Children under 9 years of age are fasted for 4 hours.
(2) No food should be taken during the testing period.
(3) Patients should avoid eating food containing pentose (fruits, jams, jellies) for 24 hours before the test.
(4) Water intake is not restricted, and the patient is encouraged to drink water during the test.
(5) The patient should not take medication for 24 hours before the test if possible. Aspirin, neomycin, indomethacin, and atropine in particular should be avoided.
(6) Gastrointestinal disturbances such as diarrhea may occur with the 25 gram dose. A 5 gram xylose dose shows a lower rate of gastrointestinal disturbances but may be less sensitive.
Dose of D-xylose:
(1) adults: 25 grams. A 5 gram dose can be if gastrointestinal side effects are to be avoided.
(2) children: 0.5 g D-xylose per kilogram body weight, up to maximum of 25 grams
Administration:
(1) Dissolve dose in sufficient water to make a 1:10 dilution (maximum 250 mL = 8 ounces).
(2) The patient drinks the entire 10% solution.
(3) 250 mL (8 ounces) of fresh water is then given to the patient to drink.
(4) 250 mL (8 ounces) of fresh water is given to the patient to drink 1 hour after the dose solution.
Specimen Collection:
(1) Urine: Before the dose of xylose is administered, the urinary bladder is emptied. All urine over the next 5 hours is collected. The urinary bladder is emptied at 5 hours after the dose and added to the collected urine samples.
(2) Plasma or serum: A single sample may be collected at 1-2 hours after the oral dose, or samples may be collected at 0, 1, 2, 3, 4 and 5 hours relative to the oral dose.
(3) The patient is kept supine for the duration of the test.
Specimen Testing:
(1) urine: Dilutions of 1:50, 1:100, 1:250 of urine are prepared. Concentrations of D-xylose in the sample are measured, recorded as mg/mL
(2) p-bromoaniline is the reagent used to form a colored compound, which can be measured in a spectrophotometer.
amount of D-xylose excreted in urine as grams per 5 hour testing period =
= (mg/mL of D-xylose in diluted urine sample) * (dilution factor) * (urine volume in liters)
where:
• mg/mL D-xylose is comparable to g/L
• dilution factor: 50 for 1:50 dilution, 100 for 1:100 dilution, 250 for 1:250 dilution
Interpretation
Normal urine levels following 25 gram adult dose
(1) adults < 65 years of age: >= 4 grams
(2) adults >= 65 years of age: >= 3.5 grams
Normal urine levels following pediatric dose: children: 16-33% of administered dose
Normal urine levels following 5 gram adult dose: > 1.2 grams in 5 hours
Normal plasma or serum levels following 25 gram adult or pediatric dose:
(1) 21-57 mg/dL at 1 hour
(2) 32-58 mg/dL at 2 hours
(3) 19-42 mg/dL at 3 hours
(4) 11-29 mg/dL at 4 hours
(5) 6-18 mg/dL at 5 hours
Normal plasma or serum levels following 5 gram adult dose: > 20 mg/dL at 2 hours
Reduced excretion may be seen in any disease interfering with small intestine mucosal absorption or renal excretion:
(1) nonpancreatic malabsorption, especially celiac disease
(2) impaired renal function
(3) Whipple's disease
(4) delayed gastric emptying
(5) bacterial overgrowth in small intestine
(6) vomiting
(7) hypothyroidism
(8) tropical sprue
(9) Crohn's disease
(10) pellegra
(11) intestinal parasites (ascariasis)
(12) immunoglobulin deficiencies
(13) surgical bowel resection
(14) dehydration
(15) decreased circulation
(16) radiation enteritis
(17) ascites
(18) increased intestinal motility
Unit conversion for plasma samples:
• conventional units reported in mg/dL
• SIU reported in mmol/L
• conversion factor for conventional to SI units: 0.0666
• conversion factor for SI to conventional units: 15.015
Unit conversion for urine samples:
• conventional units reported in grams in 5 hours
• SIU reported in mmol per 5 hours
• conversion factor for conventional to SI units: 6.66
• conversion factor for SI to conventional units: 0.15015
