Xylose Absorption

Absorption of D-xylose is a useful test in the differential diagnosis of malabsorption. D-xylose absorption is normal in patients with intraluminal maldigestion caused by hepatobiliary disease or pancreatic insufficiency. D-xylose absorption is decreased in most patients with mucosal malabsorption and in some patients with malabsorption due to bacterial overgrowth.

D-xylose is a five carbon monosaccharide present in some plants that is absorbed unchanged by the duodenum and jejunum. Renal excretion accounts for about half of the total d-xylose elimination in patients with normal renal function. Renal D-xylose clearance is approximately 87% of the glomerular filtration, reflecting net renal tubular reabsorption. Nonrenal elimination is presumably hepatic. Nonrenal clearance does not appear to be compromised until creatinine clearance falls below 30 mL/min.

Patients should be well hydrated and fast for 6 hours prior to the test. Between 08:00 and 09:00, one SST tube of blood and a urine sample are collected. Both specimens should be labeled, “control”. Adults are then given 25 grams of D-xylose orally, while children less than 12 years old are given 5 g. Urine is collected for exactly five hours after the dose is given. One SST tube of blood is drawn exactly one hour after xylose administration. Only the one-hour serum sample is used for adults with impaired renal function.

Normally, the five-hour urine collection should contain 5 grams or more of D-xylose. The one hour blood sample should have a D-xylose concentration of 25 mg/dL or more. Both the five-hour urine test and the one-hour serum test have an approximate sensitivity and specificity of 95% when used to discriminate between patients with proximal malabsorption and normal patients or patients with pancreatic insufficiency.

The D-xylose absorption test has limitations. Delayed gastric emptying, impaired renal function, urinary retention, and sequestration of D-xylose in ascitic or edema fluid may decrease urinary excretion. However, potential false positive results in the urine portion of the test are avoided by measurement of serum D-xylose concentration. The vagaries of gastric emptying can be overcome by instilling the xylose dose through a tube directly into the proximal intestine.

Serum reference values are >25 mg/dL in adults and >15 mg/dL in children less than 6 months old. Urine reference value is >4 g of xylose excreted in 5 hours in patients with normal renal function.

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