Isolates that are multi-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. Epidemiology of drug-resistant TB
A 1997 survey of 35 countries found rates above 2% in about a third of the countries surveyed. The highest rates of drug-resistant TB were in the former USSR, the Baltic states, Argentina, India and China, and was associated with poor or failing national Tuberculosis Control programmes.
Other abbreviations have been widely used (for example, the notations RIF, RFP, and RMP have all been widely used for rifampicin, and the combination regimens have notations such as IRPE, HRZE, RIPE, and IREP that are variously synonyms or near-synonyms, depending on dosage schedules), but for clarity, the semistandardized abbreviations used above are used in the rest of this article. In this system, which the World Health Organization (WHO) supports, "RIPE" is "RHZE". (Both have mnemonic potential, as tuberculosis is named after tubercles (small tubers), and a tuber can be ripe and can be a rhizome. )Drug regimens are similarly abbreviated in a semistandardised manner. The drugs are listed using their single letter abbreviations (in the order given above, which is roughly the order of introduction into clinical practice). A prefix denotes the number of months the treatment should be given for; a subscript denotes intermittent dosing (so 3 means three times a week) and no subscript means daily dosing. Most regimens have an initial high-intensity phase, followed by a continuation phase (also called a consolidation phase or eradication phase): the high-intensity phase is given first, then the continuation phase, the two phases divided by a slash.So,

2HREZ/4HR3means isoniazid, rifampicin, ethambutol, pyrazinamide daily for two months, followed by four months of isoniazid and rifampicin given three times a week.In the US only, streptomycin is not considered a first line drug by ATS/IDSA/CDC because of high rates of resistance. The WHO have made no such recommendation.