Carbamazepine

The carbamazepine blood test is requested to check the amount of carbamazepine in the blood and determine whether drug concentrations are in the desired (“therapeutic”) range. Initially, the test may be used to establish the appropriate dose for a particular individual. Depending on the results of the blood test, the dose of drug may be increased or decreased until the blood level is within the therapeutic range, Subsequent tests are then used to check that the level remains in the therapeutic range. If the individual’s health changes, or if they begin taking other drugs which may affect the level of carbamazepine, the test may be used to determine if the dose needs to be adjusted.

A doctor may use the carbamazepine test to help evaluate an individual who is experiencing side-effects or adverse reactions, or if their symptoms have recurred.

The routine carbamazepine test measures “total” carbamazepine, i.e. all the drug in the blood, both ‘free’ and bound to protein. Rarely, 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.

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 dose 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, concentration 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. This may be because concentrations are not be high enough, because 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. Side effects that may be seen at any dose but are related to higher concentrations include

• Dizziness
• Dry mouth
• Fatigue
• Uncoordinated movement
• Sleepiness
• Blurred or double vision
• Nystagmus— an involuntary movement of the eyeball

Other side effects may include:

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

Carbamazepine can also sometimes cause liver function abnormalities, low blood sodium concentrations, 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.

Some individuals of Han Chinese or Thai origin are at increased risk of a serious skin disorder (Stevens-Johnson syndrome) if prescribed carbamazepine and the drug should be avoided unless there is no alternative. The HLA-B*1502 test may be used to assess susceptibility in such individuals.

Carbamazepine should not be used in acute porphyria.

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”, when concentrations are at their lowest).

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 µmol/L. (4 - 12 mg/L is equivalent to 20 – 50 µmol/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 persistence of 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 dose 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 and is not 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.

Carbamazepine can affect or be affected by many prescribed and over-the-counter medications – such as paracetamol, warfarin, fluconazole, isoniazid, theophylline, erythromycin, , the combined oral contraceptive pill and immunosuppressant drugs (ciclosporin, sirolimus, tacrolimus). Also, the metabolism of carbamazepine can be increased by other antiepileptic drugs such as phenobarbital, primidone and phenytoin. The effect of this increased metabolism is to decrease 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. Further information on drug interactions is available on the NICE BNF web site.

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.

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 they may only need the medication for a short period of time.