"Personalized medicine" is a new concept in health care, one aspect of which defines the specificity and dosage of drugs according to effectiveness and safety for each patient. Dosage strongly depends from the rate of metabolism which is primarily regulated by the activity of cytochrome P450. In addition to the need for a genetic characterization of the patients, there is also the necessity to determine the drug-clearance properties of the polymorphic P450 enzyme. To address this issue, human P450 2D6 and 2C9 were engineered and covalently linked to an electrode surface allowing fast, accurate, and reliable measurements of the kinetic parameters of these phase-1 drug metabolizing polymorphic enzymes. In particular, the catalytic activity of P450 2C9 on the electrode surface was found to be improved when expressed from a gene-fusion with flavodoxin from Desulfovibrio vulgaris (2C9/FLD). The results are validated using marker drugs for these enzymes, bufuralol for 2D6, and warfarin for 2C9/FLD. The platform is able to measure the same small differences in K(M), and it allows a fast and reproducible mean to generated the product identified by HPLC from which the k(cat) is calculated.