Improvement and Validation of d-Xylose Determination in Urine and Serum as a New Tool for the Noninvasive Evaluation of Lactase Activity in Humans

Background The phloroglucinol assay is the current method for d‐xylose determination in urine/plasma/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). An improved assay was therefore needed. Methods We developed and validated a modified version of the phloroglucinol‐based assay for quantification of d‐xylose in urine/serum samples. A method for gaxilose determination by gas chromatography (GC) was also optimized. Results Linearity ranged from 0.125 to 5.0 mg/l (5–200 mg/l in original sample). Accuracy at LOQ (0.125 mg/l) was 0.97/2.49% in spiked urine/serum; for other quality controls (QC), it was