A Study Evaluating Conventional and Automated System of Culture for Isolation of Mycobacteria from Smear Negative Tuberculosis Samples with Special Reference to MPT 64 Antigen

BACKGROUND India is among one of the major tuberculosis (TB) endemic countries of the world. The Centers for Disease Control and Prevention (CDC) guidelines mandate early species identification of M. tuberculosis complex as an effective countermeasure. The conventional culture method, which is the ‘gold-standard’ technique of mycobacterial isolation is time consuming, while the newer, automated, liquid medium-based culture systems like BACT / ALERT 3D have considerably shorter detection time and greater sensitivity. Other than isolation it is also important to differentially identify Mycobacterium tuberculosis complex (MTBC) from nontuberculous mycobacteria (NTM). As MPT 64 antigen is specific for MTBC and can be detected with considerable accuracy it can be used as a tool to differentiate between MTBC and NTM. METHODS 200 samples (sputum, cerebrospinal fluid-CSF, pleural fluid etc.) in total were collected from clinically suspected smear negative patients. Each sample was inoculated both in L-J media and BACT / ALERT 3D system. Those samples which showed growth were further differentiated into MTBC and NTM both by conventional biochemical tests and MPT64 antigen detection kit test. RESULTS Out of the 200 samples, 30 produced growths. Lowenstein-Jensen (L-J) media detected 12 (40 %) and BACT / ALERT 3D system detected 26 (86.67 %) of the isolates. Among the 30 isolates, 18 (60 %) were MTBC and 12 (40 %) were NTM. Furthermore, L-J media detected 44.4 % and BACT / ALERT 3D system detected 88.9 % out of 18 MTBC isolates while among the 12 NTM isolates L-J media detected 33.33 % and BACT / ALERT 3D system detected 83.33 %. Mean detection time for MTBC was 46.5 days by L-J media and 20.8 days by BACT / ALERT 3D system. Mean detection time for NTM was 24.5 days by L-J media and 9.86 days by BACT / ALERT 3D system. Contamination rate in this study was 12 % in L-J media and 3 % in BACT / ALERT 3D system. We also found that for sensitivity, specificity, positive predictive value, negative predictive value of MPT64 antigen detection kit test was 100 % in our study considering biochemical tests as gold standard. CONCLUSIONS BACT / ALERT 3D system has a very good isolation rate and shorter mean detection time compared to L-J media even from smear negative samples. Also, MPT-64 antigen detection kit test is a very viable option to differentiate between MTBC and NTM. KEYWORDS MTBC, NTM, BACT / Alert 3D, MPT 64 Antigen