Creatinine (C4H7N3O), potassium phosphate monobasic (KH2PO4) and potassium phosphate dibasic (K2HPO4) were obtained from Merck (Darmstadt, Germany). Creatinine reagents (CREP2-Creatinine plus ver.2, Roche Diagnostics, Basel, Switzerland; ref. 03263991, lot no. 687743) were collected from the disposable dead volume of automatic analyser cassettes used at a local hospital in Bangkok, Thailand. The remaining reagents were pooled from ten cassettes that were collected on different days within two months after opening. The creatinine assay kits (CREATININE liquicolor) based on the Jaffe kinetic reaction were purchased from HUMAN diagnostic (Wiesbaden, Germany). L-ascorbic acid, D-(þ)-glucose, uric acid and human serum albumin were obtained from Sigma-Aldrich (St. Louis,