Operational Handbooks

The first-in-class product for low complexity automated NAATs for detection of resistance to INH and second-line anti-TB drugs is the Xpert MTB/XDR Assay (Cepheid, Sunnyvale, USA). This test uses a cartridge designed for the GeneXpert instrument to detect resistance to INH, FQs, ETO and second-line injectable drugs (AMK, kanamycin and capreomycin). However, unlike Xpert MTB/RIF and Xpert MTB/RIF Ultra, which are performed on a GeneXpert instrument that can detect six colours, the new test requires a 10-colour GeneXpert instrument.

Health needs are diverse, and programmes are expected to provide a range of diagnostics to assist health workers in managing patients. The diagnosis of TB often begins with symptom screening, which is not specific to TB, given that cough and fever overlap with COVID-19 and other respiratory infections. Additionally, people with TB may be coinfected with HIV, particularly in sub-Saharan Africa, and services for both diseases are usually provided at the same levels of care.

In selecting a diagnostic test to implement, it is important to consider the characteristics (i.e. risk factors) of the population being served. These characteristics should be derived from population-based studies, if available, and should include the proportion of:

The first-in-class product for this class is the Genoscholar PZA-TB (Nipro, Osaka, Japan) for the detection of resistance to PZA. The Genoscholar PZA-TB test is based on the same principle as the FL-LPA and SL-LPA but requires the use of a large number of hybridization probes to cover the full pncA gene (>700 base pairs [bp]). Reading the hybridization results on the crowded strips with a total of 48 probes requires careful attention to avoid errors. However, it provides faster results than phenotypic DST and is based on molecular detection.

The urine LF-LAM is an immunocapture assay based on the detection of the mycobacterial LAM antigen in urine; it is a potential point-of-care test for certain populations being evaluated for TB. Although the assay lacks sensitivity, it can be used as a fast, bedside, rule-in test for HIV-positive individuals, especially in urgent cases where a rapid TB diagnosis is critical for the person’s survival. The Alere Determine TB LAM Ag is currently the only commercially available urine LF-LAM test endorsed by WHO.

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):