Tolerance test with D-xylose

Tolerance test with D -xylose (xylose absorption test) is indicated in the differential diagnosis of malabsorption syndrome. D- xylose is a 5-carbon monosaccharide (pentose), which is about 60% passively absorbed in the proximal (duodenojejunal) part of the small intestine and is eliminated from the circulation by the kidneys. The clearance of about 87% is due to tubular resorption of D- xylose. We determine the level in serum and urine, on an empty stomach and 5 hours after the application of the load (a 5-hour urine collection is performed to determine urine waste).

Test Procedure
The patient arrives on an empty stomach, a blood and urine sample is taken, and D- xylose is administered. 25 g of D- xylose are given to adults and 5 g to children; an alternative procedure (especially in children) recommends 0.5 g / kg body weight. Saturate the patient with fluids (minimum 2 × 250 ml of tea) for 5 hours. After 2 hours another blood sample is taken, after 5 hours the last portion of urine. The total urine volume is measured after 5 hours and a sample of about 10 ml of urine is taken for analysis. The variant arrangement of the test allows blood to be taken at intervals other than 2 hours; in children, blood is usually taken as early as 1 hour.

Determination of D -xylose
The determination of D- xylose is based on the reaction of 4-bromoaniline with furfural, which is formed by the dehydration of D- xylose in an acidic medium. The resulting color product is measured photometrically at 520 nm. The reaction takes place in the presence of thiourea, which minimizes the formation of interfering color products. Blood samples must be deproteinized before D- xylose is determined, and a procedure with zinc sulfate and barium hydroxide is recommended. Alternative methods for the determination of D- xylose are gas chromatography, HPLC and the D- xylose oxidoreductase (NADP + ) enzyme method.

The clinical significance
The serum level 1 hour after D- xylose administration is 1.40–3.80 mmol / l, 2.13–3.86 mmol / l in 2 hours, 1.27–2.80 mmol / l in 3 hours, in 4 hours 0.73–1.93 mmol / l, in 5 hours 0.40–1.20 mmol / l. The pathological result is a 2-hour level in adults after administration of 25 g <1.67 mmol / l, in children after administration of 5 g <1.33 mmol / l. In a five-hour urine collection in children 5–12 years after administration of 5 g of D- xylose, the pathological result is <0.8 g / 5 hours (a wider range of normal values ​​is 0.5–1.65 g / 5 hours), in adults persons after administration of 25 g value <4 g / 5 hours (according to some authors, the finding is evaluated <5 g / 5 hours). For people over 65, the limit value drops to 3.5 g / 5 hours. When administered D -xylose by weight (especially in young children) is the normal range of xylose excreted in the urine in 5 hours 10-33% of the administered amount.

A tolerance test with D- xylose is usually indicated to confirm intestinal malabsorption in gluten enteropathy (celiac sprue), tropical sprue. The outcome of urine output is affected by renal function. A false positive result can be determined, for example, in vomiting, dehydration , myxedema , ascites , edema. Many drugs reduce the excretion of D- xylose by the kidneys - such as aminosalicylic acid, acetylsalicylic acid, digitalis, indomethacin, neomycin and others. C14- D- xylose breath test is a variant of the xylose absorption test.

Source

 * with the permission of the author taken from KOCNA, Petr. GastroLab: MiniEncyclopedia of laboratory methods in gastroenterology  [online]. © 2002. Last revision 2011-01-08, [cit. 2011-03-04]. < http://www1.lf1.cuni.cz/~kocna/glab/glency1.htm >.