This test is used to measure and monitor the amount of phenobarbital in the blood and to determine whether the drug level is within a therapeutic range. Phenobarbital is an anti-epileptic drug used to prevent seizures in people with epilepsy. This test is used to ensure that the blood level of phenobarbital is not too low so as to cause a recurrence of seizures or too high as to cause side effects. Depending on the result, a health practitioner may adjust the drug dose up or down.