XDR-TB, an abbreviation for extensively drug-resistant tuberculosis (TB), is a form of TB which is resistant to at least four of the core anti-TB drugs. XDR-TB involves resistance to the two most powerful anti-TB drugs, Isoniazid and Rifampicin, also known as multidrug-resistant (MDR-TB), in addition to resistance to any of the Fluoroquinolones (such as Levofloxacin or Moxifloxacin) and to at least one of the three injectable second-line drugs (Amikacin, Capreomycin or Kanamycin).

MDR-TB and XDR-TB both take substantially longer to treat than ordinary (drug-susceptible) TB, and require the use of second-line anti-TB drugs, which are more expensive and have more side-effects than the first-line drugs used for drug-susceptible TB.

If TB bacteria are found in the sputum, the diagnosis of TB can be made in a day or two. To confirm XDR-TB, however, it may take from 6 to 16 weeks.

XDR-TB patients can be cured, but with the current drugs available, the likelihood of success is much smaller than in patients with ordinary TB or even MDR-TB. Cure depends on the extent of the drug resistance, the severity of the disease and whether the patient's immune system is compromised.