D-Xylose Absorption (Timed Urine and Blood)

The D-xylose test is a diagnostic measure for evaluating malabsorptive conditions and intestinal absorption of D-xylose, a pentose not normally present in the blood in significant amounts. It is passively absorbed in the proximal small bowel, passes unchanged in the liver, and is excreted by the kidneys.

This test directly measures intestinal absorption. When D-xylose (which is not metabolized by the body) is administered orally, blood and urine levels are checked for absorption rates. Absorption is normal with pancreatic insufficiency but is impaired with intestinal malabsorption. It is a reliable index of the functional integrity of the jejunum in pediatric patients.

Blood

1-Hour absorption of 5-g dose—infant: >15 mg/dL or >1.0 mmol/L

1-Hour absorption of 5-g dose—child: >20 mg/dL or >1.3 mmol/L

2-Hour absorption of 5-g dose—adult: >20 mg/dL or >1.3 mmol/L

2-Hour absorption of 25-g dose—adult: >25 mg/dL or >1.6 mmol/L

Urine Xylose 5-Hour Reference Range for 25-g Dose

Child: 16%–33% of 5-g dose

Adult: >16% of 5-g dose or >4.0 g of maximum (0.5 g/kg to a maximum of 25 g)

Adult, 65 years of age and older: >14% of dose or >3.5 g of maximum

1. Have the patient refrain from foods containing pentose for 24 hours before test.

2. Do not allow food or liquids by mouth for at least 8 hours before the start of the test. Pediatric patients should fast only 4 hours.

3. Have the patient void at the beginning of the test. Discard this urine.

4. Administer the oral dose of D-xylose after it has been dissolved in 100 mL of water. Adult dosage is 25 g; for children younger than 12 years, a 5-g oral dose is recommended. For adults, additional water up to 250 mL should be taken at this time and another 250 mL in 1 hour. Record times on the patient’s healthcare record. Give no further fluids (except water) or food until the test is completed.

5. Draw a 3-mL sample of venous blood within 60–120 minutes later.

6. Have the patient rest quietly in one place until the test is completed.

7. Have the patient void 5 hours from the start of the test. Save all urine voided during the test.

1. Urine D-xylose is decreased in:

   1. Intestinal malabsorption

   2. Celiac disease

   3. Impaired kidney function

   4. Small bowel ischemia

   5. Whipple disease

   6. Viral gastroenteritis (vomiting)

   7. Bacterial overgrowth in small intestine

2. The D-xylose test is normal in the following conditions:

   1. Malabsorption due to pancreatic insufficiency

   2. Postgastrectomy

   3. Malnutrition

Pretest Patient Care

1. Explain purpose of test, procedure for urine collection, and interfering factors. The entire 5-hour specimen must be collected.

2. The patient must fast at least 8 hours before the start of the test; children younger than 9 years should fast for only 4 hours.

3. Water may be taken at any time.

4. Weigh the patient to determine the proper dose of D-xylose.

5. The patient must discontinue contraindicated medications for 1 week before specimen collection. Obtain healthcare provider’s approval.

6. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Many drugs and antibiotics (see Appendix E)

2. Nonfasting state, treatment with hyperalimentation

3. Foods rich in pentose (fruits and preserves)

4. Vomiting of the xylose test meal (25-g dose may cause gastrointestinal distress)

5. Impaired kidney function—use serum test only

6. In adults, the serum test has little value—use 5-hour urine test.