Molecular tests for TB - Genexpert & Truelab

Which are the latest molecular tests for TB?

Some of the latest molecular tests for TB are often referred to as Nucleic Acid Amplification Tests (NAATs). NAATs can detect TB and perform drug susceptibility testing (DST) for key drugs such as rifampin (RIF) and isoniazid (INH). They produce results much more quickly than a culture test.

Which tests are currently recommended by WHO?

There are several molecular TB tests that are already recommended by the World Health Organisation (WHO) and which are also commercially available.

Genexpert Platform

The Cepeid Xpert MTB/RIF

The Cepeid Xpert MTB/RIF

The first such test was the Xpert MTB/RIF assay for use on the Genexpert platform. This test  became available in 2010. An updated WHO policy was published in 2013.

In 2017 WHO recommended Xpert Ultra, the next generation of Xpert MTB/RIF as the initial TB diagnostic test for adults and children, regardless of HIV status, over smear microscopy and culture. As an automated PCR based test, Ultra can be used by minimally trained technicians. But as it runs on the Genexpert platform, it requires a continuous power supply and a computer which limits its use as a true point of care (POC) test.

The Genexpert Edge system is battery powered and utilizes a tablet, making it more portable.

Truelab group of tests

The Truenat MTB, the Truenat MTB Plus, and the Truenat MTB-Rif Dx are a group of tests made in India. The first two assays detect M. tuberculosis bacilli in sputum after extraction using the separate TruePrep instrument and kits. The Truenat MTB-Rif Dx is available as an optional add on chip for sequential RIF detection of resistance.

Truelab comes in Uno, Duo and Quattro throughput formats. It was designed to be POC friendly as it has a dust filter and it runs in temperatures up to 30 degree C. However multiple micropipetting steps mean that trained technicians are required to operate it.

WHO Recommendations

Module 3 (diagnosis of TB) of the 2020 WHO Consolidated Guidelines, recommends using Truenat MTB or MTB Plus rather than smear microscopy as an initial diagnostic test for TB in adults and children with signs and symptoms of pulmonary TB. This is a conditional recommendation.

For Drug Susceptibility Testing (DST), with a Truenat MTB or MTB plus positive result, Truenat MTB-Rif Dx may be used as an initial test for rifampicin resistance, rather than phenotypic DST. This is also a conditional recommendation.

Newest Technologies

The Xpert XDR is another cartridge designed to run on the Genexpert and Omni platforms. This will simulateously detect mutations associated with resistance to multiple first and second line TB drugs or extensively drug resistant TB. The cartridge was launched in July 2020 and a WHO review is pending.

The Genexpert was originally designed for use at the district or sub-district level.

Implementation of TB tests

The development of new tests, although very important, is not sufficient for ensuring their implementation in countries with the highest TB burdens.

"Tuberculosis innovations mean little if they cannot save lives"1Madhukar, Pai, "Tuberculosis innovations mean little if they cannot save lives", 2017, "Tuberculosis innovations mean little if they cannot save lives"

This comment was made in 2017 but it is probably equally true in 2021. It was noted that although in just a few years new diagnostics for TB had been developed, these new tools had yet to reach scale and access remained a major challenge for patients in low and middle income countries.

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This page was last updated in May 2021.
Author Annabel Kanabus

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