How worthwhile is plasma primidone level measurement?

Simultaneous steady-state plasma levels of primidone and phenobarbitone were studied in 43 patients receiving primidone therapy. Primidone and phenobarbitone levels in the individual appeared to be linearly related but steady-state plasma phenobarbitone levels correlated better with primidone dose than did steady-state plasma levels of primidone itself. This pattern of correlation is probably due to primidone being more rapidly eliminated than the phenobarbitone that is derived from it. Steady-state plasma primidone levels showed more inter-dosage fluctuation than steady-state plasma phenobarbitone levels in the same patients. In the subjects studied age, sex, and concurrent anticonvulsant therapy did not alter the relation between plasma levels of primidone or phenobarbitone and primidone dose. The study suggested that knowledge of steady-state plasma primidone levels adds little to knowledge of plasma phenobarbitone levels in guiding the therapy of epilepsy with primidone.