Evaluation of the performance of two real-time PCR assays for detecting Mycobacterium species

Rapid discrimination between Mycobacterium tuberculosis (MTB) and nontuberculous mycobacteria (NTM) is critical for patient treatment and to avoid unnecessary expenditure on infection control. Because real-time PCR assays distinguish MTB from NTM, we evaluated the performance of two real-time PCR assays (AdvanSure and PowerChek).

This study used 143 DNA samples from respiratory specimens which were collected based on routine PCR results using Anyplex kit. A total of 87 positive samples (65 MTB and 22 NTM) and 56 negative samples were collected consecutively during 6 months and 1 month, respectively. The diagnostic performance of PCR assays (AdvanSure and PowerChek) was retrospectively analyzed based on the results of conventional mycobacterial tests and routine PCR assay.

Based on culture results, the sensitivities/specificities of AdvanSure and PowerChek were 90.7%/87.6% and 92.6%/85.4%, respectively, for MTB detection. For PCR-positive specimens, the quantification cycle (Cq) values of smear-negative specimens were higher than those of the smear-positive specimens (P < 0.001). As expected, the two PCR assays had the same sensitivities for NTM detection, viz. 90.0%, and their specificities were 99.2% and 98.4%, respectively. The overall agreement rate between the three PCR assays was 96.5% for MTB and 97.9% for NTM.

The sensitivities of PCR assays in our study might be overestimated, because this study enrolled relatively lower number of PCR-negative samples which potentially missed PCR-negative but culture-positive specimens. However, the two real-time PCR assays for detecting MTB and NTM perform equally well in relative performance evaluation and their Cq values can be considered suitable for predicting smear-positive specimens.