Monitoring phenytoin concentrations, along with careful clinical assessment, is the most effective means of improving seizure control, reducing the risk of toxicity, and minimizing the need for additional anticonvulsant medication for the following reasons:
1. Phenytoin concentrations correlate better with pharmacologic activity than does dosage because of individual differences in absorption, metabolism, disease states, concomitant medication, and compliance. Concentration monitoring helps physicians individualize dosage regimens.
2. The hepatic enzyme system for metabolizing phenytoin can become saturated within the drug’s therapeutic range. When this occurs, small dosage alterations can lead to unexpected drug accumulation and clinical toxicity.
3. Phenytoin is safe and effective only in a narrow range of concentrations.
1. Manufacturer’s reagent package insert, Architect® Phenytoin, Microgenics Corporation 46500 Kato Road Fremont, CA 94538 USA, May 2015.