Optimizing a Phenytoin Dosage Regimen

At the clinic you have a new patient (U. T.) age 42 years and weight 57 kg. An inital dose of 270 mg/day was recommended. The patient was instructed to take this dose each day for a few weeks and return for a plasma sample. This plasma sample was found to contain 9.5 mg/L phenytoin. This was somewhat low so you need to calculate a new dose per day with the aim of achieving an average plasma concentration of 15 mg/L. For this calculation assume that the patient has a Km value of 4 mg/L (a population average value). Hint: Using Km = 4, calculate an estimate of Vm then recalculate a new dose to achieve an average plasma concentration of 15 mg/L.

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