Simultaneous determination of phenytoin and carbamazepine in human hair by high-performance liquid chromatography

Drug detection in human hair has been regarded as a potentially useful technique for therapeutic drug monitoring and for the assessment of therapeutic compliance. An isocratic high-performance liquid chromatography (HPLC) assay for the simultaneous measurement of phenytoin and carbamazepine in human hair was used to assess therapeutic compliance in a population of patients with epilepsy. After alkaline digestion, phenytoin and carbamazepine were separated on a reverse-phase column by using a mobile phase of acetonitrile/methanol/water (9:37:54) and monitored at a wavelength of 214 nm with methylphenyl-5-phenylhydantoin as internal standard. The assay was linear in the range from 0 to 66.66 ng/mg hair, and recoveries of both drugs at concentrations of 8.33, 33.3, and 66.66 ng/mg hair from the hair matrix ranged from 91.2 to 104.0%. inter-and intraassay coefficients of variation for both drugs (assessed at three concentrations in the calibration range) were < 10%. The limits of detection for phenytoin and carbamazepine were 2.0 and 1.33 ng/mg hair, respectively.