Phenobarbital
At a Glance
Why Get Tested?
To determine if the phenobarbital level in the blood is at a therapeutic level for an individual patient
When to Get Tested?
At regular intervals to monitor phenobarbital level in the blood and as needed to detect low or toxic levels
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
None
The Test Sample
What is being tested?
This test measures the level of phenobarbital in the blood. Phenobarbital is a barbiturate, a highly addictive, sedating drug that depresses the nervous system. Doctors usually prescribe phenobarbital to prevent seizures or to relieve anxiety. Phenobarbital is often prescribed to treat epilepsy and other seizure disorders because the drug stabilizes electrical activity in the brain.
Your doctor will monitor your response to phenobarbital 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 phenobarbital 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 phenobarbital. The following drugs can have effects with phenobarbital:
- Oral blood-thinning medications like warfarin
- Antidepressants and tricyclics, including MAO inhibitor antidepressants
- Central nervous system depressants, sedatives, hypnotics and tranquilizers
- Antihistamines
- Alcohol
- Oral contraceptives
- Corticosteroids like prednisone
- Doxycycline used to treat bacterial infections
- Griseofulvin, a drug used to treat fungal infections
- Phenytoin, another medication often prescribed to treat seizures.
It is important for you to maintain a constant level of phenobarbital in the blood within the therapeutic range. If your level is too low, you may experience seizures or anxiety. If the level is too high, you could experience side effects or even toxicity.
But maintaining a constant, therapeutic level of phenobarbital in the blood can be tricky. Phenobarbital is metabolized 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. 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.
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.
The Test
Common Questions
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Article Sources
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.
National Institutes of Health’s Medline Plus. Phenobarbital. Available online at http://www.nlm.nih.gov/medlineplus/druginfo/meds/a682007.html. Reviewed October 1, 2007. Accessed October 3, 2008.
NetdoctorUK.com. Phenobarbital. Available online at http://www.netdoctor.co.uk/medicines/10002060.html through http://www.netdoctor.co.uk. Reviewed August 1, 2007. Accessed October 3, 2008.
Warner, A. et al. Standards of laboratory practice: Antiepileptic Drug Monitoring. Clinical Chemistry 1998; 44:1085–1095. Available online at http://www.clinchem.org/cgi/reprint/44/5/1085 through http://www.clinchem.org. Accessed October 4, 2008.
Kwan, P. and Brodie, M. Phenobarbital for the Treatment of Epilepsy in the 21st Century: A Critical Review. Epilepsia 2004; 9: 1141-1149. Available online at https://www3.interscience.wiley.com/cgi-bin/fulltext/118751051/HTMLSTART?CRETRY=1&SRETRY=0 through https://www3.interscience.wiley.com. Accessed October 4, 2008.
American Society of Health System Pharmacists. Phenobarbital (Systemic). Available online at http://www.ashp.org/mngrphs/essentials/a382855e.htm through http://www.ashp.org. Revised August 2007. Accessed October 5, 2008.
San Diego Reference Laboratory. Phenobarbital. Available online at http://www.sdrl.com/druglist/phenobarbital.html through http://www.sdrl.com. Accessed October 7, 2008.
Burgess, C. ed. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th Edition, 2006. P. 2312.
Epilepsy Therapy Development Project. Children and Phenobarbital. Available online at http://professionals.epilepsy.com/medications/p_phenobarbital_children.html through http://professionals.epilepsy.com. Accessed October 5, 2008.
Emory School of Medicine, Department of Human Genetics. Seizures and Pregnancy. PDF available for download at http://www.genetics.emory.edu/pdf/Emory_Human_Genetics_Seizures_Pregnancy.PDF through http://www.genetics.emory.edu. Issued 2004. Accessed October 7, 2008.
UC San Diego Medical Center. Safety of Commonly Used Drugs in Nursing Mothers. Available online at http://health.ucsd.edu/specialties/pharmacy/resources/breastfeeding.asp through http://health.ucsd.edu. Reviewed April 2005. Accessed October 8, 2008.
Spencer, Jeanne P. et al. American Family Physician. Medications in the Breast Feeding Mother. Published July 1, 2001. Available online at http://www.aafp.org/afp/20010701/119.html through http://www.aafp.org. Accessed October 8, 2008.
Food and Drug Administration. Information for Healthcare Professionals: Suicidality and Antiepileptic Drugs. Available online at http://www.fda.gov/cder/drug/InfoSheets/HCP/antiepilepticsHCP.htm through http://www.fda.gov. Issued January 31, 2008. Accessed October 4, 2008.
Reinberg, Steve. FDA Advisers Don't Back 'Black Box' Warning for Epilepsy Drugs. The Washington Post. Available online at http://www.washingtonpost.com/wp-dyn/content/article/2008/07/10/AR2008071002527.html through http://www.washingtonpost.com. Published July 10, 2008. Accessed October 7, 2008.
