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Multi Drug Resistant Tuberculosis (MDRTB)
Multi-drug resistant tuberculosis (MDR-TB) is defined as TB that is resistant at least to INH and RMP. Isolates that are multiply-resistant to any other combination of anti-TB drugs but not to INH and RMP are not classed as MDR-TB.
As of Oct 2006, "Extensively drug-resistant tuberculosis" (XDR-TB) is defined as MDR-TB that is resistant to quinolones and also to any one of kanamycin, capreomycin, or amikacin. The old case definition of XDR-TB is MDR-TB that is also resistant to three or more of the six classes of second-line drugs. This definition should no longer be used, but is included here because many older publications refer to it.
The principles of treatment for MDR-TB and for XDR-TB are the same. The main difference is that XDR-TB is associated with a much higher mortality rate than MDR-TB, because of a reduced number of effective treatment options. The epidemiology of XDR-TB is currently not well studied, but it is believed that XDR-TB does not transmit easily in healthy populations, but is capable of causing epidemics in populations which are already stricken by HIV and therefore more susceptible to TB infection.
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