TB Diagnostic beyond culture

India recently decided to roll out a unique test across 18 sites in the country to diagnose and confirm MDR TB. Hailed as a breakthrough in TB diagnosis, the Xpert test, developed by the Foundation for Innovative New Diagnostics (FIND), US-based Cepheid Corporation and the University of Medicine and Dentistry of New Jersey has been endorsed by WHO. It has several firsts to its credit including its ability to detect multi-drug resistant TB in less than two hours demonstrating both high sensitivity (88 per cent), as well as high specificity (98 per cent),a feat unaccomplished by its predecessors till now.

An accurate test that can offer rapid detection along with reliable diagnosis had long eluded both patients and clinicians, with developing countries relying predominantly on sputum smear microscopy, a century-old technique that has outgrown its use. Plagued by low sensitivity that further drips in field settings, arduous time to obtain results (up to a month since it is culture based) the inadequacy of smear test has also been enhanced by the increasing HIV pandemic coupled with its inability to detect drug resistance. It is only pertinent then that the research community and organisations engaged in finding new and better methods for TB diagnosis look at other approaches to solve this problem.

Molecular diagnostics: Setting a new paradigm

Culture tests have been the gold standard for TB diagnosis despite the time it took for results to be obtained. While scientists have worked at obtaining faster results in even a weeks time, an increasing approach now is to rely on molecular diagnostics. The Bill and Melinda Gates Foundation has been instrumental in its support of research, development, and delivery of new TB diagnostics by distributing grants worth $150 million since towards this cause.

Elaborates Gene Walther, Deputy Director for Diagnostics, Bill & Melinda Gates Foundation, “Molecular testing for TB, has been transformational, it is the first real advance in TB diagnosis since the 1880s with an improved performance, allowing us to detect disease earlier, thereby permitting the initiation of treatment, as well as the reduction and spread of TB.” He stresses that this is not end of the road, there is still room for further improvement in molecular diagnostics to improve access to state-of-the-art diagnostic technologies.

Back in India, a company in Bangalore is determined to prove Walther right. Molbio Diagnostics, a joint venture between Bigtec Labs and Tulip group has developed a point-of-care test, Truelab, a real time PCR assay which hits the market this month. The battery operated test uses state-of-the-art Micro Electro-Mechanical Systems(MEMS) technology and can give results in an hour.

“We conducted a study with Hinduja Hospital, Mumbai, the results of which were published in PLoS One. Truelab has a sensitivity greater than 99 per cent in smear positive (S+) and culture positive (C+) cases and a specificity of 100 per cent (versus a composite reference standard (CRS)). It was also able to detect all TB strains in an evaluation study using a panel from WHO/TDR that contains over 220 geographically diverse strains of TB,” asserts, Chandrasekhar Nair, Chief Executive Officer, Bigtec labs.

While the sputum smear test showed a low sensitivity of 40-50 per cent with the rate further declining in countries with high prevalence of pulmonary TB and HIV infection, tests such as Xpert and Truelab rank high on both specificity and sensitivity and are the need of the hour. Detecting drug resistance is also a critical factor in which Xpert scores over Truelab, an area the latter is working upon. High specificity coupled with high sensitivity and detecting drug resistance, thus, have emerged as three important parameters to determine the efficacy of a test for TB diagnosis. This is further confirmed by studies over the past few years which blame the failure of TB diagnostics on either the disease being underdiagnosed (thus increasing the risk of transmission), or overdiagnosed (utilising resources of the healthcare system) or worse delayed diagnosis of drug resistance with the patient acquiring resistance and spreading the infection further.

Fitting into the big picture

Priced at $9.98 per cartridge Xpert MTB/RIF is already being scaled up in over 20 countries, with 1,900,000 cartridges procured in the public sector in 77 of the 145 countries eligible for concessional pricing. Price reduction was facilitated by Gates Foundation, UNITAID and other donors. While funding for TB diagnostics research by donor institutions is gaining momentum, the figure has remained far below that of drugs and vaccines for TB. Reluctance on the part of companies to invest in product development fearing market dynamics and a low return of interest does not help the cause either. Most of them are scared to test the waters after the government imposed a ban on serological tests for TB diagnosis citing their inaccuracy.

“An accurate antibody or antigen detection test will make a big difference to TB control, especially if it can be introduced as a rapid, dipstick or lateral flow test. Much more basic research and biomarker work is needed to make that happen,” chips in Dr Madhukar Pai, Associate Professor, Dept of Epidemiology & Biostatistics, Mcgill University.

Finding validated, accurate biomarkers then is the key. “Biomarkers offer next generation diagnosis of TB by helping identify the DNA of the bacterium rather the cellular wall (smear microscopy) which not only helps increase sensitivity, but also enables rapid diagnosis and detection of drug resistance,” adds Walther. The Gates Foundation has invested $11 million in new grants to identify biomarkers for the development of novel TB diagnostics. Use of accessible samples such as urine, saliva, breath, and perhaps blood and the ease of doing these tests outside of a formal lab setting would enable healthcare providers to screen and diagnose patients more quickly, he feels. An effective test that is affordable and accessible could, however, lose out in absence of a proper diagnostic algorithm.

An algorithm is a recommended sequence in which procedures such as symptom screens can be combined with tests in a diagnostic pathway. An efficient algorithm would integrate procedures and tests to ensure the most rapid, accurate, and rational diagnosis and treatment for all patients. India’s failure in this area can only be revealed by the fact that even after achieving 100 per cent DOTS coverage TB still remains a huge burden. While the Revised National TB Control Programme (RNTCP) has shown a lot of leadership by banning inaccurate TB blood antibody tests, making TB case notification compulsory, increasing its budget to support introduction of new innovations, including the scale up of the recent Xpert test, much more needs to be done.

“Most TB patients in India first seek care in the private sector where subsidised prices for WHO-endorsed tests like Xpert, line probe assay and liquid culture are not available. So, there is no affordable alternative to TB serology, hence the government should consider ways to make WHO-endorsed tests more affordable in the private sector,” exhorts Pai.

Initiatives such as IPAQT – Initiative to Promote Affordable and Quality TB Tests are the path bearers in this regard, he feels. Meanwhile it remains to be seen how both Xpert and Truelab would be implemented in peripheral microscopy centre labs and primary health centres where they would impact clinical outcomes even as the hunt for an innovative test that can possibly rise above all limitations is on.

shalini.g@expressindia.com