A Simple Paper-based Colorimetric Device for Rapid and Sensitive Urinary Oxalate Determinations

In this work, a simple and inexpensive paper-based colorimetric device (cPAD) was developed for oxalate measurements. The colorimetric assay is based on the formation of formazan via the reduction of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) by oxalate decarboxylase and formate dehydrogenase on the paper device. After the sample was spotted on the device, MTT changed color from yellow to purple, and the purple color intensity correlated with the oxalate concentration. The quantitative detection of oxalate ranged from 10 – 1000 μM, with a linear equation, y = 0.0086x + 34.978, and a correlation coefficient (R2) = 0.9994. The detection limit was 10 μM by the naked eye. The recoveries of oxalate spiked in artificial urine samples evaluated by our cPADs were in the range of 81 – 92%. This simple cPAD for rapid and sensitive oxalate determination should be useful for diagnostic urinary oxalate measurements for point-of-care monitoring.