The emergence of extensively drug-resistant tuberculosis: a global health crisis requiring new interventions: part I: the origins and nature of the problem.

Surveillance studies and outbreak investigations indicate that an extensively drug-resistant (XDR) form of tuberculosis (TB) is increasing in prevalence worldwide. In outbreak settings among HIV-infected, there is a high-case fatality rate. Better outcomes occur in HIV-uninfected, particularly if drug susceptibility test (DST) results are available rapidly to allow tailoring of drug therapy. This review will be presented in two segments. The first characterizes the problem posed by XDR-TB, addressing the epidemiology and evolution of XDR-TB and treatment outcomes. The second reviews technologic advances that may contribute to the solution, new diagnostics, and advances in understanding drug resistance and in the development of new drugs.