This test measures the level of phenobarbital in the blood. Phenobarbital is a barbiturate, an antiepileptic drug (AED) and sedating drug that depresses the nervous system. Doctors usually prescribe it to prevent seizures or to relieve anxiety. It is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain.
It is important to maintain a stable level of phenobarbital in the blood within the therapeutic range. If the level is too low, the person who is being treated may experience seizures or anxiety. If the level is too high, the individual could experience side effects or even toxicity.
Maintaining a constant, therapeutic level of phenobarbital in the blood can be difficult. The difference between the level at which the drug is therapeutic and the level at which toxic side effects can occur is very small. This is called a narrow therapeutic index and is a primary reason the drug requires close monitoring.