Editorial: Revolutionary test for TB


A new test developed for diagnosing active tuberculosis is set to revolutionise treatment of a disease that kills 1.8 million people round the world every year. It recently won approval from the World Health Organisation for a worldwide rollout over the next few years. The approval comes within three months of publication in the New England Journal of Medicine of the results of a trial conducted on 1,700 patients in five countries, including India. The new test has several advantages over the currently used smear microscopy and conventional nucleic acid-amplification method. While the sensitivity of smear microscopy is about 50 per cent, this (Xpert MTB/RIF) has 72 per cent sensitivity with one test, and 90 per cent with three tests in the case of smear-negative patients. The sensitivity goes up to 98 per cent in the case of smear-positive and culture-positive patients. Xpert has 99 per cent specificity. Further, the test’s ability to provide reliable results within two hours, compared with 4-6 weeks in the case of culture, will help begin treatment earlier and reduce the chances of an individual infecting others. The greatest beneficiaries will be those co-infected with HIV and TB. The long wait for the results before starting TB treatment is one of the main reasons for the death of many co-infected individuals.

Unlike smear microscopy, Xpert can identify rifampicin drug resistance. It correctly identified 98 per cent of bacteria that were resistant to rifampicin. In India and many other countries where multidrug resistant TB (MDR-TB) is not high, much of the testing that goes on now is mainly for diagnosing active TB and not for drug resistance. But Xpert is all set to change this: rifampicin resistance is an excellent marker of MDR-TB. Most patients who are resistant to rifampicin are also resistant to isoniazid drug. Patients who are resistant to rifampicin will need culture to find out which drugs work for them. Following this protocol before starting the treatment will go a long way in preventing MDR-TB from becoming widespread. There is one major problem, however: the diagnostic test is prohibitively expensive. India being one of the high-burden countries, the public sector and certain NGOs will be eligible for a special pricing agreement. Uninterrupted power supply and temperature control, which are essential, will turn out to be major challenges in rural areas. India must find ways to embrace this technology swiftly after necessary field testing — considering that TB kills two Indians every three minutes.

Published in The Hindu on December 28, 2010