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Objective: Determination of substrates by enzymatic methods often terminates with production of hydrogen peroxide and its detection in Trinder reaction, catalyzed by peroxidase. Interferences by reducing agents present in blood plasma, leading to falsely lower results have been repeatedly reported. The aim of the study was to assess the influence of common metabolites and drugs on determination of creatinine and uric acid; these analytes occur at the lowest concentration from commonly measured substrates and are therefore most susceptible to interference. Design: experimental study Settings: Institute of Clinical Biochemistry and Haematology, University Hospital in Pilsen, Czech Republic Material and Methods: Creatinine and uric acid were determined in pooled plasma samples by an enzymatic method ending with a Trider reaction. The effect of potentially interfering substances, both natural metabolites and drugs in real concentrations, has been investigated, both in plasma and by mixing with a blood sample when extracted from an intravenous cannula. For creatinine determination the influence of the interfering substance at different analyte concentrations was assessed on the metamizole interference model. Results: The largest negative biases were provided by the following drugs: dobesilate, etamsylate, N-acetylcysteine and metamizole. Bilirubin interference has been demonstrated but was clinically insignificant. Ascorbate oxidase activity present in the reagent was sufficient to eliminate interference of physiological concentrations of ascorbic acid, but the result was already affected with high dose therapy. Adrenaline, noradrenaline and dobutamine are administered diluted as infusions, therefore their effect was low or even negligible. Lactate and 3-hydroxybutyrate did not affect Trinder reaction. Conclusion: Influencing the enzymatic determination of creatinine and uric acid by certain metabolites and drugs should be considered even at their commonly occurring concentrations. [ABSTRACT FROM AUTHOR]