Improved Assay Determines d-Xylose in Urine and Serum

The phloroglucinol assay is the current method for d-xylose determination in urine, plasma, or serum; however, its sensitivity is limited when low amounts of D-xylose are to be measured, such as in the noninvasive evaluation of intestinal lactase with 4-galactosylxylose (gaxilose).

Clinical chemists at the Universidad Autónoma de Madrid (Spain) obtained 25 human urine and venous blood samples from healthy subjects who attended a local hospital. D-xylose and gaxilose determination in urine and serum after oral gavage of gaxilose was performed in six hypolactasic subjects with lactase activity in small intestine biopsy of less than 10 U/g protein enrolled in a multicenter, open label, nonrandomized trial, designed to address the diagnostic performance of the gaxilose test.

D-xylose determination was carried out using a modification of the phloroglucinol method and the absorbance in the colorimetric assay of D-xylose was read using a double beam spectrophotometer (Hitachi, Tokyo, Japan) set at 554 nm. A method for gaxilose determination by gas chromatography (GC) was also optimized.

The linearity of the improved D-xylose assay ranged from 0.125 to 5.0 mg/L as compared to 5 to 200 mg/L by the original method. Accuracy at the lower limit of quantification (LOQ), 0.125 mg/L, was 0.97%/2.49% in spiked urine or serum. For other quality controls (QC), it was less than 1.27%. Intra- and inter-assay precision at LOQ were 6.02% and 6.45% for urine, and 8.86% and 10.00%, respectively, for serum and for other QC; precision was less than 2.15%. Linearity of gaxilose determination by GC was 3.90 to 195.17 mg/L for urine and 9.75 to 95.17 mg/L for serum with acceptable sensitivity and reproducibility. The method proved adequate for the D-xylose determination in healthy and hypolactasic subjects after oral administration of gaxilose.

The authors concluded that the modified method provides high sensitivity and robustness for D-xylose quantification in urine and serum samples for routine clinical use especially in the noninvasive diagnosis of intestinal lactase deficiency with the gaxilose test. The study was published in the November 2014 issue of the Journal of Clinical Laboratory Analysis.

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