Performance evaluation of automated real-time PCR assays for diagnosis of TB in HIV infected individuals

Background: Tuberculosis (TB) remains as the leading cause of HIV/AIDS related mortality. Evidence of TB was found in about 32% and 67% of HIV infected adults during autopsy; emphasize the need for timely TB diagnosis. Due to longer turn-around time taken by conventional culture methods, faster and reliable diagnosis by direct molecular detection of Mycobacterium tuberculosis (MTB) complex DNA has gained widespread importance. This study evaluated the performance of different Real-Time PCR (RT PCR) platforms for TB diagnosis in HIV co-infected individuals from resource-limited settings. Methods and materials: A total of 40 processed sputum specimens from HIV infected individuals received for AFB culture using BD BACTEC MGIT 320, was used for this evaluation. Following AFB culture, remaining sputum specimens were tested using Real-Time PCR assays such as Cepheid Xpert MTB/RIF assay (Limit of detection: 131 CFU/mL), Abbott RealTime MTB assay (Limit of detection: 17 CFU/mL) and Roche COBAS TaqMan MTB assay (Limit of detection: 16 CFU/mL). Results: Abbott RealTime MTB assay showed a sensitivity and specificity of 100% and 87.5%, while the sensitivity and specificity of COBAS TaqMan MTB assay were 87.5% and 100% respectively, compared to MGIT 320. Xpert MTB/RIF assay showed 100% sensitivity and specificity with MGIT 320. Abbott RealTime MTB assay detected 4 MTB infections additionally while COBAS TaqMan MTB assay did not detect MTB in 1 specimen, which was detected by all the other assays studied. Conclusion: Compared to MGIT 320, Abbott RealTime MTB assay showed higher sensitivity while COBAS TaqMan MTB assay showed higher specificity. Results of Xpert MTB/RIF assay were same as that of MGIT 320. Ability of Abbott RealTime MTB assay to detect dual targets in MTB DNA sequence could have resulted in diagnosing additional TB cases compared to other RT PCR assays. Among the 4 additional TB cases detected by Abbott RealTime MTB assay, 3 were positive for pulmonary TB confirmed clinically by X-ray findings, 1 was positive for extra-pulmonary (abdominal) TB and they were not on anti-tuberculosis treatment. Considering the short turn-around time, reliability and higher sensitivity, these RT PCR assays could be used efficiently for MTB screening in TB suspected cases, especially in HIV settings.