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 stabilise the moods of patients with bipolar disease and to help alleviate some types of nerve pain. 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. Carbamazepine is prescribed to help prevent specific types of recurrent seizures.

Bipolar disorder is a mental condition that is characterised by cycles of depression and mania that may last for days, weeks, months, or years. 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 is also sometimes treated with carbamazepine.

Carbamazepine is sometimes used to treat alcohol withdrawal syndrome.

Carbamazepine levels need to be monitored because carbamazepine is a drug that has a narrow safety range. If levels are too low, the patient may experience a recurrence of symptoms (i.e. seizures, mania or pain); if levels are too high the patient may experience increased toxic side effects.

This balance 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 metabolised 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.
  • 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 until a steady concentration in the therapeutic range 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 it used?

The carbamazepine test measures 'total' carbamazepine, both protein-bound and free portions of the drug. It is ordered to measure and monitor the amount of carbamazepine in the blood to determine whether drug concentrations are in the therapeutic range.

Doctors may order 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 side-to-side movement of the eyeball

Other side effects may include:

  • A red itchy rash
  • Nausea
  • Diarrhoea
  • Constipation
  • Headache
  • Confusion

Carbamazepine can also sometimes cause liver dysfunction, low 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 seek another anti-seizure, bipolar, or nerve pain medication.

When is it requested?

Carbamazepine tests are ordered frequently at first, then at regular intervals and as needed to ensure that therapeutic blood concentrations are maintained. Additional total carbamazepine tests may be ordered if dosages of the drug are 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 stabilised, concentrations are monitored at regular intervals to ensure that they remain within therapeutic range.

The carbamazepine test may be ordered when a patient's condition does not appear to be responding to therapy. 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 ordered when a patient experiences a troublesome level of side effects and/or develops complications.

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 result mean?

The therapeutic range for carbamazepine when it is taken by itself is about 4-12 mg/L or 17-51 µmol/L. Higher levels are considered toxic. Ranges vary slightly from laboratory to laboratory. Doctors and patients should use the therapeutic ranges and units that have been established by the laboratory that performs the patient's testing.

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. Patients should work closely 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, the patient is not having recurrent seizures, mood swings or nerve pain, and the patient is not experiencing significant side effects, then the dosage of carbamazepine a patient is receiving is considered adequate. Patients should not increase, decrease or stop taking their medication without consulting with 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 acetaminophen (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 - decreasing 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 medications and supplements that you are taking.

Carbamazepine can increase the risk of certain birth defects and fetal death, and can decrease the effectiveness of oral contraceptives. Women of childbearing age should talk to their doctors about this subject.

Common questions

  • How long will I need to be on carbamazepine?

Patients who have a seizure disorder, bipolar disorder, or chronic nerve pain will typically take carbamazepine or other medications throughout their lifetime. If carbamazepine ceases to be effective or causes adverse effects, then the patient may need to be given different drug(s). If someone has seizures that are caused by a temporary condition then the person may only need the medication for a short period of time.

Last Updated: Thursday, 1st June 2023

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