Tears as the best practical indicator of the unbound fraction of an anticonvulsant drug.

Phenobarbital and carbamazepine concentrations were determined by the EMIT technique in tears, saliva, cerebrospinal fluid (CSF), and plasma of patients with epilepsy. Closer correlation was shown between tear/plasma and tear/CSF ratios than between saliva/plasma and saliva/CSF ratios for the two agents. The phenobarbital CSF/serum ratio was in good agreement with data in the literature, and the higher ratio found for carbamazepine may be caused by an EMIT assay cross-reaction for the free fraction of carbamazepine-10,11-epoxide. In our hands, tears seem to represent the best practical indicator of the unbound fraction of an anticonvulsant drug, and the noninvasiveness of the method makes it specifically useful in pediatric neurology.