A major advantage of FIA in testing clinical samples with unknown creatinine content is that the device does not require prior calibration. Instead, it incorporates internal calibration and control measurements that effectively remove the detrimental analytical effects caused by device-to-device or reagent variations. To examine the reliability and accuracy of our two-electrode FIA devices, we analyzed nineteen clinical urine samples collected in veterinary settings to determine their creatinine content and benchmarked our results against a commercial colorimetric assay (Table , Fig. ). A typical set of measurements starts with the injection of five standard creatinine solutions with known concentrations, followed by the urine samples on the same device (Fig. ). Using the internal standards, we directly determined the creatinine content in each clinical sample (Figs. and) which ranged from 7 to 47 mM, covering the entire clinical (high to low) range. Comparing our results to those benchmarked with the gold-standard commercial colorimetric kit using a laboratory plate reader (Fig. ), our paper-based two-electrode cartridges exhibit excellent agreement, with an average absolute deviation of only 11% (R 2 =0.95; Fig. ). In addition, we examined a selected panel of possible interferences at abnormally high concentrations (Table ) and apart from uric acid (+18 %), found insensitive response for all interfering compounds (<10 % signal deviation, Fig. ).