Detection of mycobacterium tuberculosis in respiratory and nonrespiratory clinical specimen using the BACTEC MGIT960 culture and DNA strand displacement amplification [SDA] assay

ABSTRACT

Direct DNA amplification techniques have provided significant tool for the diagnosis of pulmonary tuberculosis [PTB]. The efficacy of such time-saving techniques, compared to M. tuberculosis [MTB] culture in the diagnosis of extrapulmonary tuberculosis [EPTB] is improving with the invention of new nucleic acid amplification technologies. The isolation of MTB strains and the ability to obtain an anti-tuberculous drugs susceptibility profile are considered as advantages which favor the use of culture-based diagnostic assays. Automated MTB culture and antibiogram assays provide sufficient sensitivity and specificity and are timesaving in diagnosing respiratory and non-respiratory MTB infections. Since the performance of the strand displacement amplification [SDA] technique in diagnosis of PTB and EPTB lesions is being assessed worldwide, this study was performed to evaluate the use of BDProbeTec SDA [Becton Dickinson, Sparks, Maryland, USA] -recently introduced to our lab- to identify MTB in both PTB and EPTB clinical specimens and to compare it with the "gold standard" culture assays in the lab. The latter included the automated BACTEC MGIT960 System [Becton Dickinson] and subsequent culture on L-J medium followed by biochemical identification of MTB strains. Specimens were collected, prospectively, from 114 patients with clinically suspected pulmonary [n=60] and extrapulmonary tuberculosis [n=54]. The EPTB specimens were fine needle aspirates [FNA; n=30], pus [n=6], CSF [n=10], and pleural fluid [n=8]. All specimens were initially screened microscopically after ZN staining. Diff-Quik staining and microscopic examination was performed on FNA and pus specimens for cytopathologic findings suggestive of MTB lesion. MTB strains were identified by subculture on L-J medium and subsequent niacin, nitrate reduction and catalase tests. SDA identified MTB strains in 43 out of 60 PTB specimens, and 17 of 54 EPTB specimens. The sensitivity and specificity of SDA in testing PTB specimens were 93% and 88%, while in testing EPTB specimens they were 83% and 94%, respectively. Generally, SDA sensitivity was lower in AF smear-negative than AF smear-positive samples. The sensitivity [100%] and specificity [89%] of SDA when used for FNA samples were much higher than for CSF and pleural fluid. Cytopathologic examination of stained tissue/pus smears was valuable aid in the diagnosis of EPTB infections. Both SDA and culture techniques could complement each other to generate many advantages excluding MTB infection in patients with AF smear-positive samples when NTM is suspected; confirming MTB in a number of suspected samples; providing rapid diagnosis of MTB and higher sensitivity for diagnosis of EPTB in FNA specimens. On the other hand, SDA has poor sensitivity in fluid samples as pleura and CSF, and is not suitable for assessing efficiency of antituberculous drug therapy, a mission that could be successfully achieved by BACTEC MGIT automated TB culture assays