Faster, better TB test can spot drug-resistant cases too

Xpert, the quick test for tuberculosis already endorsed by WHO, has been found accurate for TB including drug-resistant strains, according to a new study publsihed in The Cochrane Library.

WHO in 2010 endorsed the two-hour, molecular diagnostic TB test, which requires little skilled expertise. With the cost of the test now reduced to under $10 per cartridge thyanks to several donors, it is being rolled out in more than 20 countries including India.

The results of the study were published in The Cochrane Library on Thursday. It made projections based on data from 18 studies worldwide involving a total of 7,816 people, mostly in low- and middle-income countries.

When the new test is used to screen 1,000 people of whom 150 have TB, it can detect 132 of the 150 cases (88 per cent) and will falsely diagnose 17 of the 1,000 (2 per cent) with TB. This is when Xpert is used as a replacement for smear microscopy.

In an alternative scenario, where Xpert is used as a replacement for culture-based, drug-susceptibility testing, it will detect the equivalent of 141 out of 150 cases (94 per cent) of rifampicin-resistant TB. Resistance to rifampicin and isoniazid is definitive of multi-drug-resistant TB. These are two of the most effective and widely used anti-TB drugs.

And when it is used as a follow-on test after conventional smear microscopy has already given a negative result, Xpert will detect 101 of 150 cases, or 67 per cent. By definition, smear-negative TB is not picked up by smear microscopy because microscopy cannot detect small numbers of bacteria.

The researchers were from the Cochrane Infectious Diseases Group in the UK, McGill University in Canada, and the Foundation for Innovative New Diagnostics (FIND). Researcher Dr Madhukar Pai of McGill told The Indian Express that their study can provide timely advice for clinicians and policymakers in countries where TB is a major public health problem.

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