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The rationale fo r measuring serum drug concentrations as a guide to therapy is based on 2 assumptions: that there is a direct relationshi p between the drug concentra tion in serum and the concentration of the free drug at the receptor site. and that this de term ines the therapeutic effect (Koch-Weser 1975). This has evolved to the curre nt target concentra tion st rategy (Sheiner & Tozer 1978: Spector et a1. 1988) in which dosage adjustments are made to bring patients' serum drug concen trations into t he ta rge t or "therapeutic' ra nge. Concentrations below or above this range are considered to be ei ther ·subtherapeutic' or ·toxic'. respectively. Recently. Spector et a1. (1988) have questioned the val ue of routine therapeutic drug monitoring in generaL citing the aminoglycosides and digoxin as specific exam ples.It has been argued that therapeutic drug monito ri ng of phenytoin is panicularly justifiable because of its narrow therapeutic range and saturable eli minat ion kinetics (Morrow & Richens 1989: Perrucca & Richens 198I a). The clinical pharmacoki netics of phenytoi n are well known. and have been reviewed b y o t hers (Mani n et a1. 1977: Richens 1979: Winter & Tozer 1986). During the past 10 yea rs. therapeutic d rug monitoring of phenytoin has become rout ine i n the management of patients with epilepsy_ The therapeutic range is generally considered t o be 40 to 80 ~mol / L. and many clinicians employ the target concent ration strategy to individualise their patients· dosages. This anicle crit ically examines the basis fo r therapeutic mon ito ring of phenytoin and the appropriateness of the target concentration strategy. It considers the pharmacokinet ic methods available for the individualisation of phenytoin dosage and, in panicular. the potential advan tagt!s of Bayesian forecasting. It also exam ines the stat us o f the debate whether free or tOlal phenytoin concentration should be monitored. Finally, the authors consider the evidence for the cli nical utility of therapeutic monitori ng of phenytoin. and the need fo r fut ure research.

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