Monitoring free extracellular valproic acid by microdialysis in epileptic patients

This report describes the first experiences with monitoring of valproic acid pharmacokinetics in patients with epilepsy by subcutaneous microdialysis. We could demonstrate good correspondence between total concentration in plasma and free concentration in plasma and dialysate. In this study the dialysate concentrations were not corrected for in vivo recovery because the aim was to explore the possibilities of the method in routine clinical work rather than to estimate absolute concentrations in extracellular fluid. In one patient, microdialysis was continued for 3 days without problems. The dialysis probe with a small portable pump could be carried without any notable interference with daily activities of patients in the ward. No side effects were noted. The results encourage further studies along two lines: to investigate the free-concentration effect relation for valproic acid and to monitor drug concentration under nonhospital conditions in order to study the interaction between daily activities and drug concentration.