Carbamazepine
At a Glance
Why Get Tested?
To determine the concentration of carbamazepine in the blood and to maintain a therapeutic level
When to Get Tested?
At the beginning of treatment to monitor the drug’s level. When indicated to detect low or high (potentially toxic) concentrations
Sample Required?
A blood sample taken from a vein in your arm
Test Preparation Needed?
No test preparation is needed.
The Test Sample
What is being tested?
This test measures the amount of carbamazepine in the blood. 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 patients with bipolar disease and to help alleviate some types of nerve pain (e.g trigeminal neuralgia). It may be prescribed by itself or in combination with other antiepileptic drugs.
Seizure disorders affect the brain’s ability to transmit electrical impulses and to regulate nerve activity. During a seizure, a patient may experience changes in consciousness, alterations in sight, smell, and taste, and may experience convulsions. Seizures are associated with acute conditions, such as high fevers and head trauma, and with chronic conditions such as metabolic disorders and brain tumours. In many cases, the cause is not known. The frequency of seizures varies from a single episode, to occasional seizures, to recurrent seizures. Seizures are categorized by the parts of the brain and body that are affected. Carbamazepine is prescribed to help prevent specific types of recurrent seizures.
Bipolar disorder is a psychiatric 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 extremely happy and excited, irritable, have poor judgment, and participate in risky behaviour. 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 are also sometimes treated with carbamazepine.
Carbamazepine levels are monitored because the drug must be maintained within a relatively narrow concentration range. If levels are too low, the patient may experience a recurrence of symptoms (i.e. seizures, mania, or pain) and if levels are too high, the patient may experience increased toxic side effects. This balance can be difficult to achieve for several different reasons:
Dosages of carbamazepine must be adjusted carefully until a steady concentration in the blood is reached. The actual amount of drug that it takes to reach this steady state will vary from person to person and may change over time.
How is the sample collected for testing?
A blood sample is obtained 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
How is it used?
The carbamazepine test measures “total” carbamazepine, i,e, all the drug in the blood, both ‘free’ and bound to protein. It is requested to check the amount of carbamazepine in the blood and determine whether drug concentrations are in the desired (“therapeutic”) range. Sometimes a free carbamazepine or a carbamazepine-10,11 epoxide test may be requested with the carbamazepine test to evaluate their contribution to the patient’s medication.
Doctors may request carbamazepine tests to help evaluate their patients for side effects and adverse reactions during initial dosage adjustments and over time. Side effects that may be seen at any dose but are related to higher concentrations include:
- Dizziness
- Uncoordinated movement
- Sleepiness
- Blurred or double vision
- Nystagmus—involuntary movement of the eyeball
Other side effects may include:
- A red itchy rash
- Nausea
- Diarrhoea
- Constipation
- Headache
- Confusion
Carbamazepine can also sometimes cause liver function abnormalities, low blood sodium levels, a decrease in white blood cells (WBCs) or an increase in eosinophils (a type of WBC). In some cases, the severity of side effects may cause the patient and doctor to select a different medication for the disorder being treated.
When is it requested?
Carbamazepine tests are requested frequently on commencing treatment, then as needed to ensure that appropriate blood concentrations are maintained. Additional carbamazepine tests may be requested if the dosage of the drug is changed, if a patient starts or stops taking additional medications (to judge their effect, if any, on carbamazepine levels), or if a patient has a recurrence of symptoms such as a seizure, nerve pain, or bipolar mood swings. Once blood concentrations of carbamazepine have stabilized, concentrations monitoring may not be necessary, but levels are sometimes monitored at regular intervals to ensure that they remain within the therapeutic range.
The carbamazepine test may also be requested when a patient’s condition does not appear to be responding to therapy and this may be because concentrations may not be high enough, the patient may not be taking the medication regularly, or the drug may be ineffective for that person. Carbamazepine tests may also be requested when a patient experiences a troublesome level of side effects and/or develops complications. A free carbamazepine test and/or a carbamazepine-10,11 epoxide test may sometimes be requested with the total carbamazepine test when a doctor wants to evaluate their contribution to drug therapy.
Patients should talk to their doctor about the timing of the sample collection. Often, the recommended time is just before the next dose is taken (“trough level”).
What does the test result mean?
The usual therapeutic range for carbamazepine when it is taken by itself is about 4 – 12 mg/L. Levels above 15 mg/L are considered toxic but ranges vary slightly from laboratory to laboratory, and may be expressed in other units such as umol/L. Doctors and patients should use the therapeutic ranges and units that have been established by the laboratory that performs the patient’s test.
Within the therapeutic range, most people will respond to the drug without excessive side effects; however, response varies with each individual. Some people will experience seizures, mood swings, or nerve pain at the low end of the therapeutic range while some people will experience excessive side effects at the upper end. Variations in free carbamazepine and carbamazepine-10,11 epoxide levels can contribute to this response. Patients should work with their doctor to find the dosage and concentration that works the best for them.
In general, if carbamazepine test results are within the therapeutic range and the patient is not having recurrent seizures, mood swings, or nerve pain, or experiencing significant side effects, then the dosage of drug the patient is receiving is considered adequate. Patients should not increase, decrease, or stop taking their medication without consulting their doctor as it can increase their risk of having a seizure and may affect other medications that they are taking. Dosage determinations and adjustments must be evaluated on a case-by-case basis.
Is there anything else I should know?
Carbamazepine can affect or be affected by many prescribed and over-the-counter medications – such as paracetamol, warfarin, fluoxetine, isoniazid, theophylline, erythromycin, ethosuximide, and benzodiazepines. Also, the metabolism of carbamazepine can be increased by drugs such as phenobarbital, primidone and phenytoin. The effect of this increased metabolism is todecrease carbamazepine concentrations in the blood. Some of these drugs may also require monitoring with blood tests. Herbal supplements, such as St. John’s wort, can also affect carbamazepine concentrations. Tell your doctor about all medication and supplements that you are taking.
Carbamazepine can increase the risk of certain birth defects and foetal death, and can decrease the effectiveness of oral contraceptives. Women of childbearing age should talk to their doctors about this.
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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.
