Drug monitoring in combination therapy

This chapter discusses the applicability of therapeutic drug monitoring during combination therapy with different antiepileptic drugs (AEDs) or during combination of an AED and drugs used for the treatment of non-epilepsy-related conditions relating to three main considerations. AED underdosage may be induced by the addition of a drug which has been indicated for a non-epilepsy-related condition. When combination therapy is associated with symptoms of intoxication, the clinical features and the electroencephalogram (EEG) are often not the right tools to ascertain which AED is responsible. The essential prerequisite for monitoring serum concentrations, both in monotherapy and in combination therapy, is the reliability of the assay methodology. The effectiveness of serum concentration determination depends on the accuracy of the therapeutic ranges of the individual AEDs. Monitoring is essential for drugs with a narrow therapeutic index and for those drugs with unacceptable incidence of toxicity.