sentences:  When MDR-TB is resistant to at least one drug from each group, it is classified as extensively drug-resistant tuberculosis (XDR-TB).In a study of MDR-TB patients from 2005 to 2008 in various countries, 43.7% had resistance to at least one second-line drug.About 9% of MDR-TB cases are resistant to a drug from both classes and classified as XDR-TB.