Pyrazinamide susceptibility testing: proposed new standard with the BACTECTM MGITTM 960 system

The susceptibility of 253 Mycobacterium tuberculosis complex isolates to pyrazinamide (PZA) was assessed using the BACTECTM MGITTM 960 (M960) system. Resistant strains underwent paired repeat testing using 1) a critical concentration of 200 g/ml (PZA-200), and 2) a reduced inoculum of 0.25 ml. They were also examined using the BACTEC 460 (B460) reference method and investigated for pncA mutations. On M960, 37 isolates were resistant. In the PZA-200 assay, 20 of these were resistant and 17 susceptible, while 18 were resistant and 19 susceptible with reduced inoculum. The B460 assay and pncA sequencing confirmed results with reduced inoculum.