Carbamazepine

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. Sometimes a free carbamazepine or a carbamazepine-10,11 epoxide test will be ordered along with the carbamazepine test to evaluate their contribution to the patient’s medication.

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 back and forth movement of the eyeball

Other side effects may include:

A red itchy rash

Nausea

Diarrhea

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.

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 stabilized, 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. A free carbamazepine test and/or a carbamazepine-10,11 epoxide test may also be ordered along with the total carbamazepine test when a doctor wants to evaluate their contribution to drug therapy. Sometimes results are evaluated as a percentage of free to total carbamazepine.

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 therapeutic range for carbamazepine when it is taken by itself is about 4 – 12 µg/mL. Levels above 15 µg/mL are considered toxic. Ranges vary slightly from laboratory to laboratory, however, and may be expressed in other units – such as mg/L. 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. Variations in free carbamazepine and carbamazepine-10,11 epoxide can contribute to this response. 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.

Carbamazepine can affect or be affected by many prescribed and over-the-counter medications such as acetaminophen, 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.

The U.S. Food and Drug Administration (FDA) announced in December 2007 that the label and prescribing information for carbamazepine is to include a recommendation that patients of Asian descent get a genetic blood test before starting therapy with carbamazepine. The genetic test, which detects variants in the HLA-B* 1502 gene, can identify a significantly increased risk of developing a rare, but serious, skin reaction. This gene variant occurs almost exclusively in people with Asian ancestry. Those who test positive should avoid being treated with carbamazepine.

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.