MDR-TB (Multidrug Resistant TB) describes strains of tuberculosis-causing bacteria that are resistant to at least the two main first-line TB drugs - isoniazid and rifampicin. XDR-TB, or Extensive Drug Resistant TB (also referred to as Extreme Drug Resistance) is MDR-TB that is also resistant to three or more of the six classes of second-line drugs.
Resistance to anti-TB drugs in populations is a phenomenon that occurs primarily due to poorly managed TB care. Problems include incorrect drug prescribing practices by providers, poor quality drugs or erratic supply of drugs, and also patient non-adherence.
Recent findings from a survey conducted by WHO and CDC on data from 2000-2004 found that XDR-TB has been identified in all regions of the world but is most frequent in the countries of the former Soviet Union and in Asia.
