Introduction: Therapeutic drug monitoring (TDM) helps to optimize the dose of antiepileptic drugs. Only limited information is available about the clinical utility of TDM of antiepileptic drugs in India. Hence, we aimed to study the clinical utility of antiepileptic TDM in a tertiary care hospital in India and to explore the association between the plasma drug levels and the occurrence of breakthrough seizures and drug toxicity.
Methods: All patients taking antiepileptic drugs for whom TDM was done from January 2008 to December 2010 were included in the study. All relevant information was obtained from patient medical records. Trough levels were measured for all drugs using chemiluminescence assay. Drug levels were interpreted as within, below, and above the reference range, as recommended by the International League Against Epilepsy guidelines.
Results: Of the 420 samples analyzed during this period, 396 samples were included in this study for analysis. The maximum number of requests was for phenytoin (50%) followed by valproic acid (26%). The most common indication for TDM was dosage adjustment (38%) followed by breakthrough seizures (34%). Among the 135 samples received with breakthrough seizures as indication, more than 50% had drug levels either within or above the reference range. Among the 62 samples referred with clinical symptoms of suspected toxicity, drug levels were above the reference range in only 52% of the samples.
Conclusions: Therapeutic drug monitoring was found to be useful in practice, in tailoring drug dosage in accordance with the needs of individual patient, in distinguishing nonresponders from noncompliants, and in aiding in making critical decisions. However, the "reference range" of these antiepileptic drugs was not reliable in predicting the occurrence of breakthrough seizures and clinical symptoms of suspected drug toxicity.