The Test Sample
What is being tested?
Carbamazepine is a drug that is primarily used to treat certain seizure disorders (also called epilepsy) but is also prescribed to stabilize the moods of people with bipolar disease, to ease alcohol withdrawal, and to help alleviate some types of nerve pain. It may be prescribed by itself or in combination with other antiepileptic drugs. This test measures the level of carbamazepine in the blood.
Carbamazepine levels are monitored because the drug must be maintained within a narrow therapeutic range. If the level is too low, the person may experience a recurrence of symptoms (i.e., seizures, mania, or pain); if the level is too high, the person may experience toxic side effects. Maintaining a therapeutic level of the drug can be a challenge to achieve for several different reasons:
- Oral doses of carbamazepine are absorbed in the gastrointestinal tract at widely variable rates.
- Since carbamazepine is metabolized by the liver, anything that affects liver function can affect blood levels of the drug.
- Much of the drug is bound to plasma protein, but it is the free portion of the drug that is active. Conditions that affect protein binding of the drug may affect therapeutic effectiveness. This may occur when someone has low blood albumin, kidney failure, or when they are very young (newborn) or elderly.
- The metabolite of carbamazepine, carbamazepine-10 11 epoxide, is also active and contributes to the overall effect of the medication.
- Several drugs, if taken in conjunction with carbamazepine, may interact or affect metabolism and blood levels.
Dosages of carbamazepine must be adjusted carefully in order to reach a steady concentration in the blood. The actual amount of drug required to reach this steady state will vary from person to person and may change over time.
Epilepsy affects the brain's ability to transmit electrical impulses and to regulate nerve activity. During a seizure, a person may experience changes in consciousness, alterations in sight, smell, and taste, and may experience convulsions. The frequency of seizures varies from a single episode, to occasional seizures, to recurrent seizures. Carbamazepine is prescribed to help prevent specific types of recurrent seizures.
Bipolar disorder is a mental condition that is characterized by cycles of depression and mania that may last for days, weeks, months, or years. During a depressive episode, those affected may feel sad, hopeless, worthless, and have thoughts of suicide. During a manic episode, those affected may be euphoric, irritable, use poor judgment, and participate in risky behaviors. Carbamazepine is prescribed to help even out the moods of the person with bipolar disorder, especially mania.
Trigeminal neuralgia, a condition associated with facial nerve pain and muscle spasms, and paroxysmal choreoathetosis, a movement disorder that causes involuntary movements of the limbs, trunk, and facial muscles, are also sometimes treated with carbamazepine. So is diabetic peripheral neuropathic pain, which affects the functionality, mood, and sleep patterns of approximately 10-20% of people with diabetes mellitus. (For more on this, see the Neuropathy article.)
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
People who take carbamazepine should talk to their healthcare provider about the timing of the sample collection. Often, the recommended time is when the level in the blood is at its lowest, just before the next dose is taken.
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.