Free level monitoring of antiepileptic drugs. Clinical usefulness and case studies.

The free fraction of phenytoin, carbamazepine and valproic acid shows considerable interindividual variability, especially in the presence of associated disease or drug interactions. When binding is altered, the total concentration no longer reflects the amount of pharmacologically active drug in the plasma: this may mislead the clinician into making inappropriate dosage adjustments. Measuring the free drug concentration eliminates a potential source of interpretative errors and may be preferentially used to monitor therapy in selected patients.