phenobarbital-phenytoin serum levels /-fen′itō″in/ , the concentration of phenobarbital and phenytoin in the serum, monitored to maintain concentrations sufficient to control seizures but not high enough to cause toxic reactions. The control of seizures is commonly obtained in adults with plasma concentrations of phenobarbital that average 10 μg/mL per daily dose of 1 μg/kg; in children, 5 to 7 μg/mL per daily dose of 1 μg/kg. The control of seizures is commonly obtained with plasma concentrations of phenytoin that average 10 μg/mL, whereas toxic effects, such as nystagmus, typically develop with a concentration of 20 μg/mL. Ataxia may develop at a concentration of 30 μg/mL and lethargy at a concentration of 40 μg/mL.
