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International audience; BACKGROUND: Voriconazole (VRC), a widely used triazole antifungal, exhibits significant inter- and intra-individual pharmacokinetic variability. The main metabolite voriconazole N-oxide (NOX) can provide information on the patient’s drug metabolism capacity. OBJECTIVES: Our objectives were to implement routine measurement of NOX concentrations and to describe the metabolic ratio (MR), and the contribution of the MR to VRC therapeutic drug monitoring (TDM) by proposing a suggested dosage-adjustment algorithm. PATIENTS AND METHODS: Sixty-one patients treated with VRC were prospectively included in the study, and VRC and NOX levels were assayed by LC-MS/MS. A mixed logistic model on repeated measures was implemented to analyze risk factors for the patient’s concentration to be outside the therapeutic range. RESULTS: Based on 225 measurements, the median and interquartile range were 2.4 μg/mL (1.2; 4.2), 2.1 μg/mL (1.5; 3.0), and 1.0 (0.6; 1.9) for VRC, NOX, and the MR, respectively. VRC C(min) 1.15 and