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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 medicine phenobarbital in your blood. Phenobarbital is used to treat epilepsy in children and adults. Epilepsy is a disease that causes seizures, including tonic-clonic, complex, partial, or myoclonic seizures.

A brand name medicine containing phenobarbital is Luminal.

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. 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 advised range, depending on your 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 need this 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:

  • Headache

  • Memory problems

  • Depression

  • Decreased sexual interest

  • Anxiety

  • Sweating

  • Confusion

  • Slurred speech

  • Dizziness

  • Nausea

  • Anemia

  • Rash

  • Fever

  • Hyperactivity in children

  • Trouble paying attention in children and adults

Some people may have an increased risk for suicide when taking phenobarbital. But this is rare. 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 trouble 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 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?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

Phenobarbital is measured in micrograms per milliliter (mcg/mL). The normal therapeutic range for adults is around 10 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 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 is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be 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. 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 illegal drugs you may use.

Online Medical Reviewer: Chad Haldeman-Englert MD
Online Medical Reviewer: Raymond Turley Jr PA-C
Online Medical Reviewer: Tara Novick BSN MSN
Date Last Reviewed: 9/1/2022
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