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Also known as: Phenobarb; phenobarbital
Formal name: Phenobarbitone

At a Glance

Why Get Tested?

To determine if the phenobarbitone level in the blood is at a therapeutic level for an individual patient taking phenobarbitone or the related medication, primidone

When to Get Tested?

At regular intervals to monitor phenobarbitone level in the blood and as needed to detect a low or toxic level

Sample Required?

A blood sample drawn from a vein in your arm

The Test Sample

What is being tested?

This test measures the level of phenobarbitone in the blood. Phenobarbitone is a barbiturate, a highly addictive, sedating drug that depresses the nervous system. Doctors usually prescribe phenobarbitone to stop or prevent seizures. Phenobarbitone is often prescribed to treat epilepsy and other seizure disorders because the drug stabilises electrical activity in the brain. A related drug, primidone, is also used for people with seizure disorders and phenobarbitone levels may be tested in people taking this medication.

Your doctor will monitor your response to phenobarbitone to make sure that you have the desired level of the drug in your bloodstream and to determine the dose that works best for you. Monitoring also helps assure your doctor that the level of the drug circulating in your bloodstream is not toxic. Your doctor might order a phenobarbitone level when you begin taking the drug, and any time while on the medication, to determine if the dose is right for you. Your doctor might also decide to order a test if you begin taking another medication because several common drugs can affect how your body responds to phenobarbitone. The following drugs can have effects with phenobarbitone:

  • other medications used to treat seizures such as phenytoin and valproic acid (sodium valproate)
  • oral anticoagulant medications like warfarin
  • antidepressants
  • central nervous system depressants, sedatives, hypnotics and tranquilisers
  • antihistamines
  • alcohol
  • oral contraceptives
  • corticosteroids like prednisone
  • doxycycline used to treat bacterial infections
  • griseofulvin, a drug used to treat fungal infections

It’s important for you to maintain a constant level of phenobarbitone in the blood within the therapeutic range. If your level is too low, you may experience seizures. If the level is too high, you could experience side effects or even toxicity.

But maintaining a constant, therapeutic level of phenobarbitone in the blood can be tricky. Phenobarbitone is metabolised by liver enzymes and eliminated in the urine at different rates, depending on your age and overall health. Depending on dose, age and health, elimination can take several days to weeks.

How is the sample collected for testing?

A blood sample is collected by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

Ask a Laboratory Scientist

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.