In the US in January 2022, the CDC recommended hybrid definitions, allowing use of the WHO definitions or alternatives appropriate in the US: Multidrug-resistant TB (MDR-TB): Resistance to isoniazid and rifampin, the two most effective first-line drugs, with or without resistance to other drugs Pre-XDR-TB: Resistance to isoniazid and rifampin and also any fluoroquinolone; CDC alternative definition: Resistance to isoniazid, rifampin, and a second-line injectable (amikacin, capreomycin, and kanamycin) Extensively drug-resistant tuberculosis (XDR-TB): Resistance to isoniazid, rifampicin, any fluoroquinolone, and at least one additional group A drug (The group A drugs are levofloxacin, moxifloxacin, bedaquiline, and linezolid.