Stock standard solutions (1.00 mg/mL) of creatinine and ascorbic acid were freshly prepared in deionized water and 5% phosphoric acid, respectively. Uric acid standard was dissolved in water (1.00 mg/mL) by adding 0.1 M sodium hydroxide solution to adjust to pH 10.35. Work solution contained 50.0 lg/mL of creatinine and uric acid was prepared from the stock standard solution. Calibration curves were prepared over the concentration of 0.0–20 lg/mL for all creatinine, ascorbic and uric acid.