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Does this test have other names?

Phenobarbital drug monitoring, phenobarbital drug level

What is this test?

This test measures the amount of the drug 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 doctor if your dose of phenobarbital is within the therapeutic range generally accepted for the drug. 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 doctor 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 drug 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 drug

  • After any change in the dose of the drug

  • After adding any other seizure drug

  • If your doctor 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 doctor suspects your level is too high. Drowsiness is the most common side effect. Other side effects may include:

  • Headache

  • Depression

  • Dizziness

  • Nausea

  • Hyperactivity in children

This is rare, but some people may have an increased risk for suicide when taking phenobarbital. Tell your doctor 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 doctor about any learning problems. Other serious side effects that you need to let your doctor know about right away include:

  • Seizures

  • Mouth or throat soreness

  • Bleeding or bruising

  • Trouble breathing or swallowing

  • Severe rash

  • Unexplained fever

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 health care provider.

Phenobarbital is measured in micrograms per milliliter (mcg/mL). The normal therapeutic range for adults is 15 to 40 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 doctor will use your blood test, along with other factors, to determine whether you need any changes to your medication.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

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 drugs 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 doctor 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.