Rehena Sarkar1, Debalina Das2, Soumick Sarkar3
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.
