Phenobarbital

Phenobarbital (Luminal) is a long-acting barbiturate that is effective in treating generalized tonic-clonic and simple partial seizures. It is also useful in seizure control associated with withdrawal of alcohol or barbiturates in dependent individuals.

Phenobarbital toxicity is characterized by CNS sedation and reduced respiratory function. Mild toxicity can occur at concentrations above 40 ug/mL and  include ataxia, nystagmus, attention loss, and fatigue. Severe symptoms can occur at concentrations greater than 60 ug/mL. Concentrations above 100 ug/mL may be fatal. 

Phenobarbital induces the hepatic cytochrome P450 metabolic pathway and affect the metabolism of itself and other drugs. This phenomenon may complicate maintenance of effective multidrug regimens. Excretion of phenobarbital is affected by urinary pH; alkalinity enhances excretion of the parent drug. Children metabolize phenobarbital more quickly than adults do and significant pharmacokinetic changes may be seen during adolescence. 

Steady State blood levels are usually reached after 2 to 4 weeks of treatment in adults and 1 to 2 weeks in children. Sampling time is not critical because of phenobarbital’s long half-life once the steady state is reached.

Serum trough levels are useful in assessing compliance or the adequacy of initial drug regimens, changes in regimen or physiologic state, or confirming clinical symptoms of intoxication. Trough levels are usually collected immediately preceding next dose. 

Phenobarbital is measured by an enzyme immunoassay. Therapeutic range (trough level) is 15-40 ug/mL. Levels  >50 ug/mL are considered critical values.  

Specimen requirement is one plain red-top or green-top tube of blood.

Reference

Foero O, et al: Successful prophylaxis of febrile convulsions with phenobarbital. Epilepsia 1972;13:279-285.