title: Prevention of false resistance results obtained in testing the susceptibility of Mycobacterium tuberculosis to pyrazinamide with the Bactec MGIT 960 system using a reduced inoculum.
abstract: The susceptibility of 211 clinical isolates of Mycobacterium tuberculosis complex (201 M. tuberculosis and 10 Mycobacterium bovis isolates) to pyrazinamide (PZA) was assessed by the nonradiometric Bactec MGIT 960 system (M960). Detection of PZA resistance was followed by a repeat testing using a reduced inoculum (RI) of 0.25 ml instead of 0.5 ml. According to the first M960 analysis, resistance was observed in 55 samples. In the RI assay, 32 samples turned out to be susceptible and 23 proved to be resistant (58.2% false positivity). The Bactec 460 assay confirmed as resistant those strains detected by the RI assay, while discrepant results were found susceptible. Mutation analysis performed on 13 M. tuberculosis isolates detected pncA mutations in 11 samples. On the basis of our data, we suggest using the RI assay to confirm all PZA resistance results obtained with the standard M960 assay. Further studies are required to confirm our findings.
authors.affiliation: Regional Reference Mycobacteria Laboratory, United Hospitals, Ancona, Italy. piersim@tin.it
authors.forename: Claudio
authors.initials: C
authors.lastname: Piersimoni
authors.forename: Alessandro
authors.initials: A
authors.lastname: Mustazzolu
authors.forename: Federico
authors.initials: F
authors.lastname: Giannoni
authors.forename: Stefano
authors.initials: S
authors.lastname: Bornigia
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Gherardi
authors.forename: Lanfranco
authors.initials: L
authors.lastname: Fattorini
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D005189:False Positive Reactions; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D009163:Mycobacterium bovis / Q000187:drug effects* / Q000302:isolation & purification; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000302:isolation & purification; D011718:Pyrazinamide / Q000494:pharmacology*; D014376:Tuberculosis / Q000382:microbiology
chemical_list: D000995:Antitubercular Agents; D011718:Pyrazinamide