Evaluation of the Cobas TaqMan MTB Test for the Detection of Mycobacterium tuberculosis Complex According to Acid-Fast-Bacillus Smear Grades in Respiratory Specimens

Abstract

We evaluated the performance of the Cobas TaqMan MTB test (Roche Diagnostics, Basel, Switzerland), stratified by acid-fast bacilli (AFB) smear grades. The sensitivity of this test in smear-positive specimens was >95% in all grades, while that in trace and negative specimens was 85.3% and 34.4%, respectively. T he rapid detection of Mycobacterium tuberculosis complex DNA from respiratory specimens and the ability to differentiate M. tuberculosis complex from nontuberculous mycobacteria (NTM) are important for the early diagnosis of pulmonary tuberculosis and the prompt use of adequate antibiotics (1-3). The direct detection of M. tuberculosis complex DNA by PCR-based assays has become an important part of the rapid diagnosis of tuberculosis (4). Numerous molecular assays for the rapid detection of M. tuberculosis complex DNA have been developed. The Cobas TaqMan MTB test (Cobas MTB test) (Roche Diagnostics, Basel, Switzerland) is one of the most widely used realtime PCR assays. It uses TaqMan hydrolysis probes and primers that bind to a highly conserved and specific region of the 16S rRNA sequence.Recently, various rates of detection by different PCR methods, based by smear grade, were noted (5-7). The aim of this study was to investigate whether this observation extends to the Cobas MTB test and to investigate its diagnostic accuracy when stratified by acid-fast bacilli (AFB) smear grades.This study was conducted at a tertiary care hospital in Seoul, South Korea, and was approved by the institutional review board of that hospital. A total of 6,852 Cobas MTB test results for respiratory specimens from April 2013 to June 2014 were retrospectively reviewed. Microbiological tests, including AFB smear and mycobacterial culture, were simultaneously performed for all specimens.The respiratory specimens were processed with 2% N-acetyl-Lcysteine-sodium hydroxide (NALC-NaOH), followed by centrifugation at 3,000 ϫ g for 20 min. The AFB smears were performed with an auramine-rhodamine fluorescent stain and subsequently confirmed by Ziehl-Neelsen staining. The staining results were graded according to the American Thoracic Society/Centers for Disease Control and Prevention (ATS/CDC) guidelines, as follows (8): no AFB seen, no bacilli in 300 fields; trace, 1 to 2 bacilli in 300 fields; 1ϩ, 1 to 9 bacilli in 100 fields; 2ϩ, 1 to 9 bacilli in 10 fields; 3ϩ, 1 to 9 bacilli in 1 field; and 4ϩ, Ͼ9 bacilli in 1 field. The specimens with the no AFB seen and trace grades were defined as smear negative, and those graded 1ϩ to 4ϩ were defined as smear positive. All patient specimens were cultured on both solid and liquid medium for 6 weeks. The positive cultures were confirmed by both the presence of cord formation and by MPT64 antigen testing (SD Bioline TB Ag MPT64 rapid assay; Standard Diagnostics, Inc., Yongin, South Korea). If any of these tests yielded a negative result, an rpoB-specific PCR test using the MTB-ID V3 kit (YD Diagnostics, Yongin, South Korea) was performed to differentiate between M. tuberculosis and NTM. The Cobas...

Performance evaluation of the Cobas TaqMan MTB assay on respiratory specimens according to clinical application

“…One recent meta-analysis confirmed that the Cobas assay exhibits heterogeneous performance (Horita et al, 2015). This heterogeneity may be due to AFB smear status, variable specimen types, and baseline incidence of TB (Huh et al, 2015;Jonsson et al, 2015). In particular, the present authors have previously found that the performance of the Cobas assay depends on smear status (Huh et al, 2015).…”

“…Many recent studies have assessed the diagnostic performance of the Cobas assay (Bloemberg et al, 2013;Huh et al, 2015;Jonsson et al, 2015;Lee et al, 2013;Yang et al, 2011). Various results have been obtained in these studies.…”

“…Many studies have characterized the performance of the Cobas assay, with results varying from study to study (Bloemberg et al, 2013;Horita et al, 2015;Huh et al, 2015;Jonsson et al, 2015;Kim et al, 2011;Yang et al, 2011). Some studies have suggested that this variance is due to the acid-fast bacilli (AFB) smear status, variable specimen types, and incidence of TB (Huh et al, 2015;Jonsson et al, 2015). However, no study has yet addressed the performance of this assay in the context of different clinical purposes, and more specifically for diagnosis when TB is strongly suspected versus when TB is considered to be less likely.…”

Clinical and Laboratory Differences between Lymphocyte- and Neutrophil-Predominant Pleural Tuberculosis

“…All positive cultures were subjected to an AFB smear to confirm the presence of AFB and to exclude contamination. The Cobas TaqMan MTB test (Cobas MTB test) (Roche Diagnostics, Basel, Switzerland), which is a real-time PCR assay, was used to analyze sputum and pleural effusion samples [18]. …”

Bacterial Diseases

“…PCR has been widely used to detect mycobacterial nucleic acid in environmental and patient samples and has a number of advantages, including speed and sensitivity. A variety of PCR assays have been described to distinguish MTC from NTM and to further speciate the latter [14,15]. Caution is advised in interpreting PCR results as false positive and negative results are possible.…”