Compliance and variability of plasma phenytoin levels in epileptic patients

Plasma phenytoin concentrations from 74 epileptic patients were evaluated for variability among three or more samples drawn within a steady-state period (SSP). Of 175 SSP, 148 were associated with supervised or probably compliant drug ingestion, and the mean coefficient of variation (CV) was 6.8 ± 3.9%. A CV of greater than 20% was associated with noncompliance. These results indicate that during steady state, variability of phenytoin concentrations attributable to fluctuations of phenytoin clearance within an individual is small and that noncompliance may be detected from the magnitude of the CV.