Rudi Hayden
Urologist
Blood-tests Explained
Disclaimer:
This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion
Phenobarbital
At a Glance
Why Get Tested?
To monitor the phenobarbital level in your blood to ensure a therapeutic level while avoiding toxic side effects
When to Get Tested?
At the start of therapy and at a regular intervals during treatment; when someone has signs and symptoms of toxicity or is experiencing seizures
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.