What is Carbamazepine/Tegretol (TDM)?
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 drug may not be effective and 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 this 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.
- Upon a long-term administration, carbamazepine induces its own metabolism.
- Much of the drug is bound to plasma protein, but it is the free portion of the drug that is active. Conditions that affect the 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 therapeutically active and contributes to the overall effect of the medication.
- Several drugs, if taken in conjunction with carbamazepine, may interact or affect its 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.
Carbamazepine is usually monitored long-term because it is prescribed to treat the chronic conditions epilepsy, bipolar disorder, trigeminal neuralgia, and nerve pain from diabetes.
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 it used?
The carbamazepine test is used to measure and monitor the amount of carbamazepine in the blood to determine whether the level of the drug is within the therapeutic range. Carbamazepine is primarily used to treat certain seizure disorders (also called epilepsy) but is also prescribed to stabilize the moods of people with bipolar disease, ease alcohol withdrawal, and help alleviate some types of nerve pain.
Initially, the test may be used to establish the appropriate dose for an individual. Depending on the results of the blood test, the dose of the drug may be adjusted up or down until the blood level reaches the therapeutic range. Subsequent tests are then used to ensure that the level remains in the therapeutic range. If a person’s health changes or if, for example, they begin taking other drugs, the test may be used to determine whether an adjustment to the prescribed dose is necessary.
A healthcare practitioner may use a carbamazepine test to help evaluate someone who is experiencing side effects or adverse reactions or a recurrence of symptoms.
A routine carbamazepine test measures total carbamazepine, both protein-bound and free portions of the drug. Sometimes a test to measure free carbamazepine or the biologically active metabolite, carbamazepine-10,11-epoxide, is ordered along with the total carbamazepine test to evaluate their contribution to a person’s therapy.