Does this test have other names?
Phenobarbital drug monitoring, phenobarbital drug level
What is this test?
This test measures the amount of the medicine phenobarbital in your blood. Phenobarbital
is used to treat epilepsy in children and adults. Epilepsy is a disease that causes
brain seizures or convulsions. Phenobarbital may be used to treat different types
of seizures, including tonic-clonic, complex, partial, or myoclonic seizures.
Other names for medicines containing phenobarbital include Luminal and Mysoline.
If you take phenobarbital to keep seizures under control, the amount of phenobarbital
in your blood needs to be at the right level. This level is called the therapeutic
range. The therapeutic range has a low number, or concentration, and a high number.
If your blood level is less than the low number you may be more likely to have a seizure.
If it is above the high number, you may be more likely to have side effects.
This test tells your healthcare provider if your dose of phenobarbital is within the
therapeutic range generally accepted for the medicine. This range is an average, though,
and the therapeutic range for you is the amount of phenobarbital that controls your
seizures with the fewest side effects. Your provider may prescribe a dose of phenobarbital
that goes over the generally recommended range, depending on your particular condition.
If you're an older adult, you may be more sensitive to this medicine and may need
a lower dose.
Why do I need this test?
You may have this blood test to make sure that the dose of phenobarbital you are taking
is within the right range for you. You may have this test:
A few days and a few weeks after starting the medicine
After any change in the dose of the medicine
After adding any other seizure medicine
If your healthcare provider prescribes another medicine that can interact with phenobarbital
If you continue to have seizures even though you are taking phenobarbital
If you become pregnant
You may also need this blood test if your therapeutic range changes over time:
When you first start taking phenobarbital, you may need to take a higher dose.
If you have been taking phenobarbital for a long time, you may need a higher dose
to get the same effect.
Your dose of phenobarbital may need to be decreased if you are older than 70.
You may need this blood test if you have side effects from phenobarbital or if your
healthcare provider suspects your level is too high. Drowsiness is the most common
side effect. Other side effects may include:
This is rare, but some people may have an increased risk for suicide when taking phenobarbital.
Tell your healthcare provider right away if you have any thoughts of suicide or if
you feel depressed. Children who are taking phenobarbital may have difficulty learning.
If your child is taking phenobarbital, tell your provider about any learning problems.
Other serious side effects that you need to let your provider know about right away
What other tests might I have along with this test?
You may also have other blood tests to check your liver, heart, and digestive system.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
Phenobarbital is measured in micrograms per milliliter (mcg/mL). The normal therapeutic
range for adults is 15 to 30 mcg/mL. If your blood level is too high, you may need
to have your dose lowered. If your blood level is too low, you may need to have your
dose increased. The therapeutic range is only a guide. Your healthcare provider will
use your blood test, along with other factors, to find out whether you need any changes
to your medicine.
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Many commonly used medicines can interfere or interact with phenobarbital, including
birth control pills.
How do I get ready for this test?
You don't need to prepare for this test, But be sure your healthcare provider knows
about all medicines, herbs, vitamins, and supplements you are taking. This includes
medicines that don't need a prescription and any illicit drugs you may use.