Diagnosis

The TB-LAMP assay is designed to detect MTBC directly from sputum specimens. This is a manual assay that provides results in less than 1 hour, does not require sophisticated instrumentation and can be used at the peripheral health centre level, given biosafety requirements similar to those for sputum-smear microscopy. TB-LAMP does not detect resistance to anti-TB drugs.

The Truenat MTB and MTB Plus assays use chip-based realtime micro-PCR for the semiquantitative detection of MTBC directly from sputum specimens and can report results in under an hour. The assays use automated, battery-operated devices to extract, amplify and detect specific genomic DNA loci.

The assays are designed to be operated in peripheral laboratories with minimal infrastructure and minimally trained technicians, although micropipetting skills are required. A practical guide is available to assist implementers considering these tests (16).

The Xpert MTB/RIF Ultra assay (hereafter called Xpert Ultra) uses the same GeneXpert platform as the Xpert MTB/RIF test, and was developed to improve the sensitivity and reliability of detection of MTBC and RIF resistance. To address sensitivity, Xpert Ultra uses two multicopy amplification targets (IS6110 and IS1081) and a larger PCR chamber; thus, Xpert Ultra has a lower LoD than Xpert MTB/RIF (16 colony forming units [cfu]/mL and 131 cfu/mL, respectively).

The Xpert MTB/RIF assay is a cartridge-based automated test that uses realtime polymerase chain reaction (PCR) on the GeneXpert platform to identify MTBC and mutations associated with RIF resistance directly from sputum specimens in less than 2 hours. WHO recommends using the Xpert MTB/RIF test in the following situations (7):

In many high TB burden settings, sputum-smear microscopy remains the primary diagnostic technique for evaluating individuals presenting with the signs and symptoms of TB. However, sputum-smear microscopy is a relatively insensitive test, with a limit of detection (LoD) of 5000–10 000 bacilli per millilitre of sputum. Furthermore, sputum-smear microscopy cannot distinguish drug-susceptible strains from drug-resistant strains. WHO recommends that TB programmes transition to replacing microscopy as the initial diagnostic test with mWRDs that detect MTBC.

The update of this operational handbook was led by Nazir Ahmed Ismail, Carl-Michael Nathanson, and Alexei Korobitsyn, with support from Cecily Miller and Matteo Zignol, and under the overall direction of Tereza Kasaeva, Director of the World Health Organization (WHO) Global TB Programme (WHO/GTB).

1. Resolution 73/3: Political declaration of the high-level meeting of the General Assembly on the fight against tuberculosis. New York: United Nations General Assembly; 2018 (https://www.who.int/publications/m/item/political-declaration-of-the-un-general-assembly-high-level-meeting-on-the-fight-against-tuberculosis).
2. Resolution 78/L.4.

New molecular assays – the Truenat MTB, MTB Plus and MTB-RIF Dx assays (Molbio Diagnostics, Goa, India), hereafter referred to as Truenat – were developed in India, and may be used at the same health system level as Xpert MTB/RIF. Of the above-mentioned assays, MTB and MTB Plus are used as initial diagnostic tests for TB, whereas MTB-RIF Dx is used as a reflex test to detect rifampicin resistance for those with positive results on the initial Truenat tests.