FRIDAY, Nov. 7 (HealthDay News) -- Extensively drug-resistant tuberculosis (XDR-TB), is becoming more common and more deadly than previously thought, new research shows.

People with XDR-TB are three times more likely to die than patients with other forms of multi-drug-resistant tuberculosis (MDR-TB), according to the findings, published in the second November issue of the American Journal of Respiratory and Critical Care Medicine.

Researchers reviewed medical records of more than 1,400 patients in South Korea with both types of tuberculosis. MDR-TB patients who didn't respond to ofloxacin and at least one second-line injectable drug were classified as having XDR-TB.

In the study, more than half the XDR-TB patients died within seven years, a rate similar to that of deaths from untreated TB in South India, the researchers said.

"[Having] XDR-TB was the strongest predictor of both all-cause and TB-related mortality, and survival curves showed higher cumulative mortality among patients with XDR-TB than in other patients with MDR-TB," study principal investigator Dr. Tae Sun Shim, an associate professor at Asan Medical Center in Seoul, South Korea, said in an American Thoracic Society news release.

The researchers found that XDR-TB patients tended to be significantly older than MDR-TB patients, and more likely to have been treated multiple times for tuberculosis in the past.

Based on their findings, the researchers classified MDR-TB as "a major threat to public health" to South Korea, where new tuberculosis cases rose from 1.6 percent to 2.7 percent between 1994 and 2004.

More information

The U.S. Centers for Disease Control and Prevention has more about tuberculosis.