Definition of Multidrug-resistant tuberculosis (MDR-TB):
A disease caused by Mycobacterium tuberculosis strains with resistance to at least isoniazid and rifampicin.
Definition of Extensively drug-resistant tuberculosis (XDR-TB):
A disease caused by Mycobacterium tuberculosis strains resistant to fluoroquinolone and at least 1 of 3 injectable second-line drugs (ie,
capreomycin,
kanamycin and amikacin) as well as both isoniazid and rifampicin (Fig.1).
Fig.: This diagram shows relationship between MDR-TB and XDR-TB. As aggressive form of MDR-TB, XDR-TB is included in MDR-TB.
Epidemiology and Clinical Significance of MDR-TB and XDR-TB:
There were an estimated 390 000–510 000 cases of MDR-TB (primary and acquired) arising in 2008,
with the best estimate at 440 000 cases.
Among all incident TB cases globally,
3.6% (95% CI: 3.0–4.4) are estimated to have MDR-TB (Tab.1).
WHO region |
Estimated number of MDR-TB cases(primary and acquired) in 2008(95% confidence interval)
|
African |
69 000 (53 000–110 000) |
Americas |
8 200 (7 300–9 300) |
Eastern Mediterranean |
24 000 (11 000–81 000) |
European |
81 000 (73 000–90 000) |
South-East Asia |
130 000 (110 000–170 000) |
Western Pacific |
120 000 (100 000–140 000) |
Total |
440 000 (390 000–510 000) |
Estimated number of MDR-TB cases (primary and acquired) in 2008,
by WHO region
References: Multidrug and extensively drug-resistant TB (M/XDR-TB)
2010 GLOBAL REPORT ON SURVEILLANCE AND RESPONSE,
WHO
XDR-TB has been reported worldwide. As of January 2010,
58 countries had reported to WHO at least one case of XDR-TB (Fig.2).
According to a recent report,
XDR-TB was shown to compose 2% of TB isolates and 9.9% of MDR-TB.
Fig.: Distribution of countries and territories reporting at least one case of XDR-TB as of January 2010
References: Multidrug and extensively drug-resistant TB (M/XDR-TB), 2010 GLOBAL REPORT ON SURVEILLANCE AND RESPONSE, WHO
XDR-TB has also been suggested to be related to higher treatment failure and mortality rate than non-XDR MDR-TB. In one previous study,
survival was significantly worse among XDR-TB cases compared with both MDR-TB cases (P=.04) and drug-susceptible TB cases (P<.001) (Fig.3).
Fig.: Survival Among XDR-TB, MDR-TB, and Drug-Susceptible TB Cases, United States, 1993-2005
References: JAMA, November 12, 2008—Vol 300, No. 18