Evidence-based Implementation of Free Phenytoin Therapeutic Drug Monitoring

Background: The majority of laboratories measure total phenytoin concentration for therapeutic drug monitor-ing. However, there are substantial interindividual vari-ations in free phenytoin concentrations, the pharmaco-logically active component. Methods: We describe the process and data used to implement monitoring of free phenytoin only in an urban medical center. Over a 6-week period, total and free phenytoin concentrations were measured, clinical charts reviewed, and indications for alterations in the percentage of free phenytoin fraction were determined. Results: Of the 189 phenytoin requests from 139 pa-tients, 136 data points were analyzed. Free phenytoin concentrations were 6.8–35.3%, with 50 % outside the expected range of 8–12%. Clinical indications likel