D-Xylose Absorption

Purpose

• To aid differential diagnosis of malabsorption
• To determine the cause of malabsorption syndrome

Patient Preparation

• Tell the patient that this test helps evaluate digestive function by analyzing blood samples and urine specimens after ingestion of a sugar solution.
• Explain that he must fast overnight before the test and that he'll have to fast and remain in bed during the test.
• Explain that several blood samples will be taken; tell him who will perform the venipunctures and when.
• Advise him that he may experience some discomfort from the needle punctures and the tourniquet but that collecting each sample takes less than 3 minutes.
• Inform him that all his urine will be collected for 5 or 24 hours.
• Withhold medications that alter test results, such as aspirin and indomethacin. Record any medications the patient is taking on the laboratory slip.

Procedure And Posttest Care

• Perform a venipuncture to obtain a fasting blood sample. Collect a first­voided morning urine specimen. Label these specimens, and send them to the laboratory immediately to serve as a baseline.
• Give the patient 25 g of D-xylose dissolved in 8 oz (240 ml) of water, followed by an additional 8 oz of water. If the patient is a child, administer 0.5 g of D-xylose per pound of body weight, up to 25 g. Record the time of D-xylose ingestion.
• For an adult, draw a blood sample 2 hours after D-xylose ingestion; for a child, I hour after ingestion. Collect the sample in a 10-ml red-top tube. Occasionally, a 5-hour sample may be drawn to support the findings of the 1-or 2-hour sample.
• Collect and pool all urine during the 5 or 24 hours after D-xylose ingestion.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Observe the patient for abdominal discomfort or mild diarrhea caused by D-xylose ingestion.
• Tell him that he may resume his usual diet and any medications withheld for the test.

Precautions

• The patient must have adequate renal function for the absorption and excretion of D-xylose.
• Handle the sample gently to prevent hemolysis.
• Tell the patient not to contaminate the urine specimens with toilet tissue or stool.
• Be sure to collect all urine and refrigerate the specimen during the collection period.
• Because patients age 65 and older and those with borderline or elevated creatinine levels tend to have low 5-hour urine levels but normal 24-hour levels, the doctor will have to establish the length of the collection period. At the end of the collection period, send the urine specimen to the laboratory immediately.
• Maintain bed rest and withhold food and fluids (other than D-xylose) throughout the test period.

Reference values

Normal values are as follows:

• Children: blood concentration> 30 mg/dl in I hour; urine, 16% to 33% of ingested D-xylose excreted in 5 hours
• Adults: blood concentration 25 to 40 mg/dl in 2 hours; urine, > 4 g excreted in 5 hours

Abnormal Findings

Depressed blood and urine D-xylose levels most commonly result from malabsorption disorders that affect the proximal small intestine, such as sprue and celiac disease. Depressed levels may also result from regional enteritis involving the jejunum, Whipple's disease, multiple jejunal diverticula, myxedema, diabetic neuropathic diarrhea, rheumatoid arthritis, alcoholism, severe heart failure, and ascites.

Interfering Factors

• Failure to adhere to pretest restrictions
• Aspirin (decreased D-xylose excretion by the kidneys)
• Indomethacin (decreased intestinal D-xylose absorption)
• Failure to obtain a complete urine specimen or to collect blood samples at designated times
• Intestinal overgrowth of bacteria, renal insufficiency, or renal retention of urine (possible drop in urine levels)