Tuberculosis is an endemic disease of great antiquity. It has conquered the entire world and has not spared any race or creed. Tuberculosis (TB) is a major public health problem in India. India accounts for one-fifth of the global TB incident cases. Each year nearly 2 million people in India develop TB, of which around 0.87 million are infectious cases. It is estimated that annually around 330,000 Indians die due to TB. TB is one of the leading causes of mortality in India killing 2 persons every three minutes, nearly 1,000 every day. Early diagnosis well supported by a Mycobacteriology laboratory equipped to perform culture, identification and drug susceptibility testing of Mycobacterium tuberculosis from clinical specimens is vital in the management of tuberculosis patients. The automated systems have improved the speed of isolation but there is a need for rapid identification of MTB isolates. Novel technologies for rapid identification of the culture isolates and Anti tubercular drug resistant isolates has become a top priority not only in TB Research but also for diagnosis & treatment purposes.
The modern molecular methods are not economically suitable for resource poor laboratories. A cheap, rapid & reliable identification test method will be of enormous help in resource poor countries. The new rapid Immunochromatographic methods have been found to be such ideal diagnostic aid in TB control programme
The main aim of this study was to evaluate a rapid & economically feasible test which accurately identifies MTB isolates grown in culture [1]. Clinically and therapeutically differential identification of M. tuberculosis from mycobacteria other than M. tuberculosis (MOTT) is very important. Most of the Mycobacteriology laboratories identify MTB using conventional biochemical tests. These tests are not labor sensitive & need special bio safety equipments. Biological, molecular and immunological studies of MTB complex have resulted in identification of different useful antigens, some of which are specific to MTB complex. Tuberculosis MPT64 also termed as protein Rv1980c is one such antigen [5, 6]. It is a protein secreted by actively growing MTB strains [7]. The MPT 64 antigen is absent in BCG strains, M. bovis & M. leprae and in other Mycobacterial species. This has been confirmed by Cloning and sequencing of MPT64 gene of H37Rv culture filtrate [8]. The ongoing MTB research studies have proved the immunogenic property of MPT64 [9, 10]. MPT64 antigen and culture filtrate protein (CFP-2) antigens are restricted antigens of MTB complex and are found to be TB specific candidate antigens [11]. It has also been proved that MPT 64 antigen is found only in Viable & actively dividing cells of M. tuberculosis[5]. Acommercial ICT test kit (Capilia TB kit) was evaluated for Rapid Identification of 784 culture isolates by employing Accu probe -MTB as the reference method for MTB identification & comparative evaluation of the rapid kit. The sensitivity of the rapid test kit was found to be 99.2%. (381/384). An important observation in this study was no false positive results were detected. Thus the study found the specificity of the rapid ICT test as 100% & have concluded that the Capilia TB ICT test kit is an useful method for rapid & routine identification of MTB isolates[3, 12]. Comparative evaluation studies of Rapid ICT test kits & conventional biochemical tests with Accu probe sequencing has proved that these ICT's have 100% specificity & a sensitivity of 97%. The Rapid ICT's are economically cheaper & the turnaround time for specifically identifying MTB is markedly reduced [4]. SD MPT 64 TB Ag rapid ICT kit with its Very high sensitivity, high specificity, no false positivity & the low tech rapidity scores over the molecular methods[1]. The sensitivity, specificity, positive predictive & negative predictive values of the SD AgMPT64 kit was found to be 97%, 100%,100% & 92% respectively [13]. No false positive or false negative results were detected in the study. ICT's detecting MPT64 antigen could be the replacement for conventional identification tests in the days to come [1]. The simplicity of the method, low cost compared to molecular methods for confirmation of MTB isolates makes the rapid ICT's an excellent choice for use in TB diagnostic laboratories. In the present study the sensitivity of SD Ag MPT 64 kit was 100%. All 54 MTB isolates showed same band similar to H37Rv culture. None of the non mycobacterial species & the non tubercular mycobacterial species tested showed Band formation for MPT64 antigen & absence of this antigen in other non tubercular isolates indicated the specificity of MPT 64 antigen for MTB only.
The positive & Negative predictive values in the study was 100%,. A lower NPV (92%) was observed in one of the study which found 100% PPV [13]. The lower NPV can only be explained as false negative result. In contrast 100% NPV (260/260) was detected in one of the study [14]. There are reports of false negativity in ICT methods among few genetically confirmed MTB strains. This is attributed to mutation occurring in the specific gene of MTB isolates [15]. One study using Capilia TB ICT kit detected false negativity in 6 MTB isolates [4]. Genomic analysis showed mutation in all 6 strains. This is a common feature encountered with both commercial kits.
There was no difference in the predictive values, sensitivity & specificity for isolates grown in liquid culture medium and on L.J. Medium. The intensity of the band was more prominent with the liquid cultures. During the study an attempt was made to evaluate the ICT for identification of MPT 64 antigen in smear positive sputum samples. Further improvement in antigen extraction method may help in employing SD MPT 64 TB Ag Rapid kit for antigen detection in clinical samples. Similar observations have been expressed by other researchers. Cost effective analytical studies of SD MPT64 TB Ag ICT and other rapid ICT's in comparison with molecular methods and culture combined with conventional biochemical tests have shown that SD MPT 64 TB Ag ICT is more economical than the other two methods. This test does not require any sophisticated equipments or specialized trained persons [16].