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 current gold standard method for the bacteriological confirmation of TB is culture using commercially available liquid media. However, culture is not used as a primary diagnostic test in many high-burden countries because of the cost, the infrastructure requirements (biosafety level 3 [BSL-3] or TB containment laboratory) and the long time required to generate results (1–3 weeks for a positive result and up to 6 weeks for a negative result). Nonetheless, conventional microscopy and culture remain necessary to monitor a person’s response to treatment. Culture is still important in the diagnosis of paediatric and extrapulmonary TB from paucibacillary samples, and in the differential diagnosis of non-tuberculous mycobacteria (NTM) infection.
The culture process can result in the growth of many of the Mycobacterium species. As such, laboratory confirmation of TB requires testing of the recovered mycobacteria using a species identification test (e.g. Capilia TB-Neo® from Tauns Laboratories, Numazu, Japan; TB Ag MPT64 Rapid Test© from SD Bioline, Kyonggi-do, South Korea; or TBcID© from Becton Dickinson Microbiology Systems, Sparks, USA) to definitively identify MTBC. Species identification is particularly important before initiating phenotypic DST.
Indirect phenotypic DST on solid (LJ, 7H10 agar, 7H11 agar) and liquid media (7H9 broth, BACTEC Mycobacterial Growth Indicator Tube™ [MGIT] system) is reliable and reproducible, and it remains the reference standard for many anti-TB medicines (Web Annex C). Reliable phenotypic DST methods are available for RIF, INH, FQs, PZA, BDQ, LZD, AMK, STR CFZ, DLM, Pa and CS. Phenotypic DST is not recommended for EMB, ETO, prothionamide, para-aminosalicylic acid, imipenem-cilastatin and meropenem.