xylose tolerance test
Last reviewed 01/2018
D-xylose is a pentose sugar which when ingested, is absorbed from the jejunum and excreted, largely unchanged, in the urine. Absorption of xylose is a measure of the intestine's ability to absorb monosaccharides.
No standard protocol is defined. A 5 g bolus is widely used being taken with 500 ml of water after an overnight fast. Absorption is assessed from urine specimens collected over a 5 hour period or alternatively, to avoid the problems of poor renal function, from blood xylose measured 1-2 hours after ingestion.
- serum xylose concentration at 1 hr in excess of 1.3 mM
- urinary xylose excretion in excess of 7.0 mmol/5 hr
Impaired absorption and excretion occurs in patients with disease of the small intestine but may also result from bacterial colonisation. The test is performed less often because of the availability of jejunal biopsy, but may be used to monitor response to therapy e.g. gluten withdrawal in coeliac disease patients.