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The long?acting barbiturate phenobarbital is an active metabolite of primidone. It is useful in seizure control associated with withdrawal of alcohol or barbiturates in dependent individuals. Its principle disadvantage is sedation, which may be a serious side effect in growing children. Phenobarbital can induce hepatic metabolism of itself and various other drugs. This phenomenon may complicate maintenance of effective multi?drug 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. 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.

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. Trough levels are usually collected immediately preceding next dose.

Therapeutic range (trough level) is 20?40 ug/mL. Levels > 50 ug/mL are considered critical values.

Specimen requirement is one plain red top tube of blood.

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