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Also known as: [Often referred to by brand name (see MedlinePlus Drug Information)]
Formal name: Phenobarbital

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The Test Sample

What is being tested?

Phenobarbital is a long-acting barbiturate, an anti-epileptic drug (AED) and sedating drug that depresses the nervous system. Health practitioners usually prescribe it to prevent seizures or to relieve anxiety. It is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain. This test measures the level of phenobarbital in the blood.

It is important to maintain a stable level of phenobarbital in the blood within the therapeutic range. If the level is too low, the person who is being treated may experience seizures or anxiety. If the level is too high, the individual could experience side effects or even toxicity. The toxic effects include drowsiness, confusion, and lack of voluntary coordination of muscle movement (ataxia), which can affect things like driving performance. Those who are chronically treated may also develop tolerance to the sedative effects of the drug and become dependent.

Maintaining a constant, therapeutic level of phenobarbital in the blood can be difficult. The difference between the level at which the drug is therapeutic and the level at which toxic side effects can occur is very small. This is called a narrow therapeutic index and is a primary reason the drug requires close monitoring.

Furthermore, phenobarbital is broken down (metabolized) by liver enzymes and eliminated in the urine at different rates, depending on a person's age and overall health. Depending on dose, age and health, elimination can take several days to weeks. Once the body has reached its capacity to metabolize phenobarbital, small increases in dose can result in large increases in levels of the drug in the blood. Side effects can become more severe and toxicity may occur.

A healthcare provider will monitor an individual's response to phenobarbital to make sure that the desired level of the drug is maintained in the blood and to determine the dose that works best for the person treated. The health practitioner might order a phenobarbital level at the start of treatment and any time while the person is on the medication to determine if the dose is right. The practitioner might also decide to order a test if a person begins taking another medication because several common drugs can affect how the body responds to phenobarbital.

Drugs that can have effects with phenobarbital include warfarin, antidepressants, sedatives, hypnotics, tranquilizers, antihistamines, alcohol, oral contraceptives, corticosteroids like prednisone, and other anti-epileptic medications such as phenytoin. In addition, phenobarbital may affect the metabolism of the above co-administered drugs and dose adjustment may needed.

How is the sample collected for testing?

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

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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

No test preparation is needed.