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.

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”).

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.

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.