At a glance
Also known as
Why get tested?
To determine if the phenobarb level in the blood is at a therapeutic level for an individual patient taking phenobarb or the related medication, primidone
When to get tested?
At regular intervals to monitor phenobarb level in the blood and as needed to detect a low or toxic level
A blood sample drawn from a vein in your arm
What is being tested?
This test measures the level of phenobarb in the blood. Phenobarb is a barbiturate, a highly addictive, sedating drug that depresses the nervous system. Doctors usually prescribe phenobarb to stop or prevent seizures. Phenobarb 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 phenobarb levels may be tested in people taking this medication.
Your doctor will monitor your response to phenobarb 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 . Your doctor might order a phenobarb 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 phenobarb. The following drugs can have effects with phenobarb:
- other medications used to treat seizures such as phenytoin and valproate (sodium valproate)
- oral medications like warfarin
- central nervous system depressants, sedatives, hypnotics and tranquilisers
- oral contraceptives
- corticosteroids like prednisone
- doxycycline used to treat infections
- griseofulvin, a drug used to treat infections
It’s important for you to maintain a constant level of phenobarb 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 .
But maintaining a constant, therapeutic level of phenobarb in the blood can be difficult. Phenobarb is by liver 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.
How long will it take for the medicine to work?
It may take a number of weeks to find the right dose, and even more time for your doctor to know how well the medicine works to control your seizures. How long this takes will be different for each person. Factors affecting dose optimisation include how often you have seizures, what other medicines you may be taking, and how your body responds to phenobarb.
How long will my phenobarb level have to be monitored?
In general, your levels will need to be monitored as long as you are taking the medication. Phenobarb, like other anti-epileptic drugs, is usually taken every day (sometimes several times a day) for your lifetime. An exception to this may be patients whose seizures are caused by a temporary condition; they may only need the medication for a short period of time.
Will this medicine affect other medicines that I am taking?
Be sure to tell your health care providers the names of all prescription medicines, herbal or dietary supplements, vitamins and over-the-counter medicines you take. Some of these may interfere with how phenobarb works, lowering or raising the level in your blood. Phenobarb may also affect how other medicines work.