The World Health Organization classifies drug-resistant TB into the following five categories:
- Isoniazid-resistant TB
- RR-TB (resistant to rifampin)
- MDR-TB (multi-drug resistant TB; resistant to isoniazid and rifampin)
- Pre-XDR-TB (pre-extensively drug-resistant TB; resistance to rifampicin plus any fluoroquinolone)
- XDR-TB (extensively drug-resistant TB; resistant to rifampicin, plus any fluoroquinolone, plus at least one of the drugs bedaquiline or linezolid)
Note: The terminology, TDR-TB (totally drug-resistant TB) nomenclature is not well-defined or currently recognized by WHO.
Global burden:
Approximately 3–4% of newly diagnosed cases are MDR-TB or RR-TB, while those previously treated have remained at 17%. In former Soviet Union countries, this percentage of drug resistance among previously treated is >50%.
123 WHO countries have reported XDR-TB. Approximately 6.2% of MDR-TB cases have XDR-TB. A small proportion of cases falls into this category, mainly due to a lack of diagnostic capacity in low and lower-middle-income countries.
History of resistance:
- In 1992, an outbreak of MDR-TB in New York City was reported. After public health intervention and control, the overall incidence of TB began to subside in the United States (US). CDC data showed a consistent decline in both drug-susceptible and drug-resistant strains. However, reports from the US indicated a gradual increase in cases of MDR-TB in foreign-born patients.
- In 2010, the appearance of several cases of XDR-TB in the US caused concern, as the cases demonstrated resistance to first-line drugs and many of the second-line drugs (fluoroquinolones, capreomycin, amikacin, and kanamycin as reported by CDC). This number has increased to 93 cumulative cases from 1993–2022. XDR-TB is rare in the US, with only 5 cases reported in 2022.2
- By 2011, India reported cases that were completely resistant to all available anti-tuberculosis drugs. Patient isolates were phenotypically resistant to the available first- and second-line anti-TB drugs, defined by their susceptibility tests (12-drug panel). Methods of diagnosis and susceptibility testing were inconsistent. In addition, many patients were co-infected with HIV.
The map to the right, presented with permission from the WHO, shows the estimated worldwide TB incidence in 2022.3
