Extensively drug-resistant TB (XDR-TB), also known as Extremely Drug-Resistant TB, is emerging as an even more ominous threat. XDR-TB is defined as TB that is resistant to any fluoroquinolone, and at least one of three injectable second-line drugs (capreomycin, kanamycin, and amikacin), in addition to isoniazid and rifampin. This makes XDR-TB treatment extremely complicated, if not impossible, in resource-limited settings.
In a 2006 XDR-TB outbreak in KwaZulu-Natal, South Africa, 52 of 53 people who contracted the disease died within months. It is estimated that 70% of XDR-TB patients die within a month of diagnosis.
The most recent drug-resistance surveillance data issued by the WHO estimates that an average of roughly 5 percent of MDR-TB cases are XDR-TB. Estimating the incidence of XDR-TB is extremely difficult because most laboratories are ill-equipped to detect and diagnose it; it is thought that the majority of XDR-TB cases go undocumented.