Xylose Tolerance Test (Fasting + Two-hour Blood + Five-hour Urine-Adult)

Evaluate possible enterogenous malabsorption syndromes; test for functional integrity of the jejunum.

This test was developed, and its performance characteristics determined, by LabCorp. It has not been cleared or approved by the Food and Drug Administration.

Xylose is a pentose found naturally occurring in certain fruits such as plums. It is not normally found in significant concentrations in blood or urine thereby rendering the duodenojejunal absorption of d-xylose a useful mechanism to assess the integrity of the gastrointestinal mucosa.

d-Xylose is passively absorbed by the small bowel with some 30% to 40% of the ingested dose rapidly excreted in the urine. Absorption into the blood is evaluated at specific time intervals with intestinal malabsorption being indicated by low absorption values. Low values would likewise be seen in celiac diseases, tropical sprue, Crohn's disease, immunoglobulin deficiency, pellagra, ascariasis, blind loop syndrome, radiation enteritis, surgical bowel resection, vomiting, delayed gastric emptying, inadequate hydration, decreased circulation, intrinsic renal disease, thyroid disease, ascites, and increased intestinal motility from any cause. The d-xylose absorption test may be of some help in distinguishing pancreatic insufficiency from jejunal malabsorption. Essentially normal d-xylose absorption values will be obtained in malabsorption values due to pancreatic insufficiency, whereas low blood values will be obtained in jejunal malabsorption.

Urine values for xylose excretion are useful in determining renal insufficiency. Diminished excretion of d-xylose could be indicative of renal insufficiency, thereby invalidating the blood xylose absorption results. Low urine xylose values could also be attributed to incomplete collection or urinary retention.

May cause mild diarrhea.

Patient must fast a minimum of eight hours prior to administration of d-xylose (LabCorp N° 26599, containing 25 g d-xylose). Patient must remain in supine position for duration of test except during urine collection. No food is permitted during the test. Patient should refrain from eating foods containing pentose and medication especially aspirin, neomycin, colchicine, indomethacin, or atropine for 24 hours prior to the test. These include fruits, jams, jellies and pastries containing these items. No water restriction; in fact, patient should be encouraged to drink during the fasting period and during test. Start test in the AM. Instruct patient to void completely. Discard this urine. Draw fasting blood specimen. Weight-based dosage of d-xylose for oral administration: 0.5 g/kg body weight up to a maximum of 25 g. Dissolved in water 10% (w/v) with a maximum of 250 mL. Have patient drink entire amount. Fill cup with 250 mL water and have patient drink this also. Have patient drink another cup with 250 mL water after one hour. Collect urine for five hours after administration of d-xylose. Record total five-hour urine volume and send aliquot. Record total on the request form. Draw remaining blood specimens (adults: two hours).