Evaluation of the BacT/Alert PZA test for susceptibility testing of Mycobacteriumtuberculosis to pyrazinamide in comparison with BacTec 460TB

Abstract number: 1733_1497

Garrigó M., Moreno C., Aragon L., Coll P.

The importance of rapid availability of Mycobacteriumtuberculosis susceptibility testing is universally acknowledged. Although BACTEC 460TB susceptibility testing is considered the gold standard, its major disadvantages are that it is semiautomatic and uses radioactive material, with the need for disposal of radioactive waste.

Objectives: The purpose of this investigation was to evaluate the performance of the BacT/ALERT PZA (Bio-Mèrieux) kit for testing M. tuberculosis susceptibility to Pyrazinamide (PZA) drug compared to the radiometric BACTEC 460TB PZA test (Becton-Dickinson)

Methods: A total of 50 M. tuberculosis isolates from human samples were included in this study. Twenty, from a culture collection of the Mycobacteria Study Group of Barcelona, were resistant to PZA having a pncA mutation. The selected clinical strains were tested in parallel with BacT/ALERT PZA kit and BACTEC 460TB PZA test. Two reference control strains, ATCC 27294 (PZA sensitive) and ATCC 35745 (PZA resistant) were included in order to validate the conditions in which the clinical strains were analysed. Control and clinical strains were prepared and tested according to the respective test procedure and the reading and interpretation of each test were performed in accordance with the manufacturer's recommendations. Strains with discrepant results were repeated in the two methods.

Results: No technical problems were detected. Five strains needed to be repeated: two because of initial discrepant results, two because of lack of growth in BACTEC 460 and one because of no respect of the test validation criteria of the BacT/ALERT PZA kit. Final overall agreement between the two systems was 100%. The total turnaround time was 13.87 days for BacT/ALERT PZA kit versus 12.39 days for BACTEC 460TB PZA test.

Conclusion: Final overall agreement between the two systems was 100%. That is, there were no differences between the two systems in the ability to detect the sensitivity to PZA or the expression of the different resistance molecular mechanisms included. There was no significant difference in the total turnaround time of the two systems. The main advantages of BacT/ALERT PZA kit are that it is fully automated with a nonradiometric detection and that it is less labour-intensive than the BACTEC 460TB system.