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Primary treatment regimens for Mycobacterium tuberculosis complex often include isoniazid, rifampin, ethambutol, and pyrazinamide (PZA). Susceptibility testing of each Mycobacterium tuberculosis complex isolate against these first-line antimycobacterial agents is a key component of patient management.
The Clinical and Laboratory Standards Institute (CLSI) provides consensus protocols for the methods, antimycobacterial agents, and critical concentrations of each agent to be tested in order to permit standardized interpretation of Mycobacterium tuberculosis complex susceptibility test results. Current recommendations indicate that laboratories should use a rapid broth method in order to obtain Mycobacterium tuberculosis complex susceptibility data as quickly as possible to help guide patient management. According to the CLSI, resistance can be confirmed by another method or by another laboratory at the discretion of the testing laboratory.
This test uses an FDA-cleared commercial system for rapid broth susceptibility testing of Mycobacterium tuberculosis complex against PZA at 300 mcg/mL. Since the literature indicates that broth testing of PZA can, at times, produce falsely resistant results, resistance to PZA by the broth method is automatically confirmed by pncA DNA sequencing. The pncA gene of Mycobacterium tuberculosis complex is responsible for activation of the prodrug PZA and hence PZA activity. Mutations in the pncA gene and upstream promoter region have been reported to account for the majority (70%-97%) of PZA-resistant isolates. However, 3% to 30% of PZA-resistant isolates do not have a corresponding pncA mutation and other genes (eg, rpsA) may also play a role.
A separate test is available for testing of the other first-line agents (isoniazid, rifampin and ethambutol).
Susceptibility testing of Mycobacterium tuberculosis complex isolates growing in pure culture against pyrazinamide
Confirming Mycobacterium tuberculosis complex resistance to pyrazinamide
Mycobacterium tuberculosis complex isolates are reported as susceptible or resistant to pyrazinamide at the critical concentration.
For resistant organisms, confirmatory testing using pncA DNA sequencing is automatically performed and the presence or absence of pncA mutations associated with pyrazinamide resistance is reported.
In vitro susceptibility does not guarantee clinical response. Therefore, the decision to treat with a particular agent should not be based solely on the antimicrobial susceptibility testing result.
Susceptibility testing should be performed on pure culture isolates of Mycobacterium tuberculosis complex.
Some mutations associated with pyrazinamide resistance that may occur outside of the pncA promoter and gene region and may therefore not be confirmed by DNA sequencing of this target.
Results are reported as susceptible or resistant.
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