The Use of Blood Levels of Antiepileptic Drugs in Clinical Practice

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Abstract
Measurement of blood levels of antiepileptic drugs has been carried out for over a decade. In earlier years, mostly spectrophotometric and colonmetric methods were used to determine diphenylhydantoin and phenobarbital concentrations. In more recent years, gas-liquid chromatographic methods have been employed which generally offer greater specificity and allow determination of several drugs simultaneously. Ceneral experience from the determination of antiepileptic drug blood levels indicates that there is some correlation between (1) the drug dose and blood level, (2) blood level and therapeutic effects, and (3) blood level and some toxic effects. The relationships are evident and usually reproducible in any individual patient. However, there are marked variations in these relationships among different patients due to variations in their individual-specific pharmacokinetic characteristics and/or external factors. Thus a wide range of blood levels may be seen in patients receiving the same dose and a wide range of clinical effects may be seen in patients having the same blood level. Nevertheless, an expected range of blood levels from a given dose can be defined, as well as an expected range of blood levels in which toxic or therapeutic effects are usually seen in the majority of patients. These expected values are clinically useful in that they caution the physician to look for specific causes in those patients whose blood levels fall far above or below the expected range. WHAT DOSAGE REGIMEN TO USE The majority of antiepileptic drugs are eliminated slowly and cause relatively stable blood levels to be maintained as long as the drug intake is constant.

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