D-xylose absorption test

Introduction

Evaluates carbohydrate absorption by the proximal small intestine.

Indications

• diagnosis of malabsorption

Contraindications

• useless in anephric patients Normal values:
• adult
  • serum
    • 25 g dose: > 25 mg/dL
    • 5 g dose: > 20 mg/dL
  • urine
    • 25 g dose: > 4.0 g/hour (> 3.5 g/hour if > 65 years)
    • 5 g dose: > 1.2 g/hour
• children
  • serum: > 30 mg/dL
  • urine: 16-33% of ingested dose

Increases

• alcohol

Decreases

• intestinal malabsorption of carbohydrates
  • normal in pancreatic insufficiency
• bacterial overgrowth in the small intestine
• vomiting
• ascites
• delayed gastric emptying
• Whipple's disease
• urine levels may be decreased with:
  • renal insufficiency
  • hypothyroidism
• pharmaceutical agents
  • aminosalicylic acid
  • arsenicals
  • colchicine
  • digitalis
  • ethionamide
  • gold
  • indomethacin
  • isocarboxazid
  • kanamycin
  • MAO inhibitors
  • metformin
  • nalidixic acid
  • neomycin
  • opium alkaloids
  • phenelzine
  • phenformin
  • atropine

Specimen

• whole blood (NaF/oxalate)
• urine 5 hour; collect in dark, refrigerated bottle

* validity of the test is enhanced if blood & urine measurements are performed simultaneously to rule out renal retention; in patients with renal insufficiency use serum levels only

Procedure

• adults:
  • 25 g oral dose of D-xylose (in 250 mL of water)
  • 5 g oral dose may cause less gastrointestinal disturbance but the test is less sensitive
  • D-xylose is measured in serum 2 hours later
  • urine is collected for 5 hours
• children
  • 0.5 g/kg orally as 5% solution (maximum 25 g)
  • D-xylose is measured in serum 1 hours later
  • urine is collected for 5 hours

More general terms

• special chemistry test

Additional terms

• malabsorption
• xylose