WASHINGTON (Reuters) - A genetic test can be used to identify patients of Asian ancestry who are more likely to develop a life-threatening skin reaction carbamazepine, a drug used to treat epilepsy and other conditions, U.S. health officials said Wednesday.

Manufacturers of drugs that contain the carbamazepine as the active ingredient have agreed to add to the prescribing instructions a recommendation that patients of Asian descent get a blood test for a specific genetic variation before beginning treatment.

The prescription medicines are Carbatrol, Equetro and Tegretol, which are used to treat seizures, bipolar disorder and nerve-related pain.

The move is another step toward "personalized medicine," in which doctors tailor treatments to specific patients.

Patients with an inherited variation of an immune system gene called HLA-B*1502 have an increased risk of rare but potentially fatal skin reactions, the Food and Drug Administration said.

These include toxic epidermal necrolysis and Stevens-Johnson Syndrome, emergency medical conditions that require immediate hospitalization. Symptoms may begin with flu-like symptoms with multiple lesions of the skin and mucus membranes, blistering, itching and fever. The allergic reaction may progress to tongue swelling, skin shedding, internal organ damage, sepsis and death.

In countries with mainly white populations, the FDA said, the chance of developing such a reaction is estimated at between 1-to-6 per 10,000 new users of carbamazepine. In some Asian countries, the risk is thought to be about 10 times higher, according to the FDA.

About 5 percent of patients being considered for carbamazepine treatment are of Asian descent, the FDA said. They can have a blood test to check for the genetic variation, which is found almost exclusively in people with Asian ancestry.

Patients who test positive should not be treated with carbamazepine "unless the benefit clearly outweighs the increased risk," the FDA said.

The drugs already carried a warning that all patients who start taking carbamazepine face a risk of rare but serious skin reactions.

The warning will now be moved to a "black box" that appears on the prescribing instructions. The recommendation on genetic testing also will be included in the black box.

Patients who have taken carbamazepine for more than a few months and have not had any skin reactions are unlikely to ever experience those problems, regardless of ancestry or genetic test results, the FDA said.

Anyone taking carbamazepine who is concerned about skin reactions should not stop taking the drug without first consulting a doctor, the FDA added.