Enzyme-based color bar-style lateral flow strip for equipment-free and semi-quantitative determination of urinary oxalate

Urolithiasis is a highly prevalent disease with increasing incidence worldwide, which occurs mainly as calcium oxalate. Therefore, measuring urinary oxalate for the early screening of urolithiasis is of growing significance. However, current approaches for oxalate detection are generally based on specific equipment, thus limiting their point-of-care applications. In this study, a colorimetric lateral flow strip was developed based on enzyme catalysis, and a color bar strategy was utilized for equipment-free and semi-quantitative determination of urinary oxalate, which does not require any image acquisition or processing equipment. Rapid semi-quantitative detection of oxalate is achieved by directly counting the number of color bars that is positively correlated with oxalate concentration. Three colorimetric patterns were compared and optimized, among which the one with 3,3',5,5'-tetramethylbenzidine (TMB) scribes presented the best results with minimized drag impact, washing effects and limited inter-reagent reactions in the lateral flow. Under optimized conditions, the assay showed a detection limit less than 0.3 mM and a detection range of 0.3-0.7 mM that covers the urinary oxalate concentrations for urolithiasis screening. Finally, the strips were verified for real urine sample analysis, vali-dating its potential for oxalate detection and urolithiasis screening in point-of-care applications.