Genetic Test Predicts Risk of Severe Rash While Taking Carbamazepine

Carbamazepine (also known by its trade name Tegretol or, for extended release, Equetro) is one of the most widely used drugs for the treatment of epilepsy, and is relatively underutilized in the treatment of bipolar disorder. One of the reasons is fear of a rare serious rash or other side effects.

The risk of the serious rash ranges from about one in 5,000 to one in 10,000. Loss of white blood cells that fight infection (a condition called agranulocytosis) occurs in about one in 20,000 people taking carbamazepine, while a decrease in white blood cells, red blood cells, and platelets (aplastic anemia) occurs in about one in 100,000 patients.

There is no way of predicting who will develop the blood disorders in reaction to carbamazepine use. A patient should contact their doctor and get a white blood cell count if they develop some warning signs of these conditions, such as a fever or sore throat without other explanation or signs of bleeding or red spots under the skin (called petechiae) that could indicate low platelets.

Genetic Test for Risk of Rash

A genetic test is available that can help estimate the likelihood of the serious rash among certain populations. In those of Asian descent, particularly Han Chinese, Thai, Malaysian, and Indian populations, having a version of the gene HLA-B known as HLA-B*1502 is highly associated with developing the rash. (The odds ratio was 79.84 in a 2013 meta-analysis by Tangamornsuksan et al. in the journal JAMA Dermatology).

In those of northern European or Japanese descent, having a version of the gene HLA-A known as HLA-A*3101 is associated with the severe rash. (Odds ratio for developing the most severe rash was 25.93 in a study of Europeans published by McCormack et al. in the New England Journal of Medicine in 2011 and 10.8 in a study of Japanese published by Ozeki et al. in the journal Human Molecular Genetics in 2011). This HLA-A*3101 gene is present in about 2 to 5% of Europeans and 9% of Japanese.

A mild, non-serious rash with redness and itchiness occurs in about 5 to 10% of patients taking carbamazepine, and almost always goes away quickly upon stopping the drug. For patients taking carbamazepine who develop any rash, stopping the drug is the safest and most conservative thing to do. However, those who have taken the HLA test who know they do not have the risk genes and have only the benign rash might want to consider continuing to take the drug.

Benefits of Carbamazepine

There are a number of reasons why carbamazepine may be worthy of a treatment trial in patients with bipolar disorder who are not doing well on other agents. Carbamazepine works well in many patients with bipolar illness who have some of the common clinical predictors of a poor response to lithium. These include: having dysphoric (anxious, irritable) rather than euphoric mania, having an anxiety or substance disorder comorbidity, having had many prior episodes or rapid cycling (four or more episodes/year), not having distinct episodes with a period of wellness in between, having a sequential pattern of depression followed by mania followed by a well interval (D-M-I rather than M-D-I), having a schizoaffective disorder with delusions or hallucinations that persist after a manic or depressive episode has ended, and having no family history of mood disorders (especially bipolar disorder).

Some patients who do not respond to another anticonvulsant such as valproate do respond to carbamazepine. Patients with bipolar depression who have had a prior history of alcoholism may also do particularly well on carbamazepine. A benefit of the long-acting version of carbamazepine called Equetro is that it can be taken at bedtime and thus help with sleep and minimize daytime side effects.

Editor’s Note: Carbamazepine induces liver enzymes called CYP3A4 that increase the metabolism (breakdown) of carbamazepine and other drugs. Several drugs that inhibit 3A4 (such verapamil and erythromycin) prevent the breakdown of carbamazepine, causing blood levels of the drug to increase and produce side effects. If you are taking carbamazepine, tell your pharmacist so he or she can monitor any other drugs you are taking for potential interactions with carbamazepine.

Knowing about the rare skin and blood side effects of carbamazepine and some of the clinical predictors of a good response to the drug may be helpful in determining whether the potential benefits of carbamazepine outweigh the risks.

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