Dr Sunita Kapoor, Director & Laboratory head, City X Ray & Scan Clinic discusses the challenges in TB diagnosis
Globally, tuberculosis is a leading cause of illness and death. It is believed that 25 per cent of the world’s population is infected with Mycobacterium tuberculosis, which has a 5-10 per cent lifetime probability of progressing to TB illness. Early identification and appropriate treatment is critical for reducing the disease impact worldwide.
Highly cost-effective antibiotic therapy for tuberculosis is widely accessible; nonetheless, the inability to recognise patients promptly means that tuberculosis remains a severe hazard to public health. Tuberculosis is an infectious airborne illness, and the WHO estimates that every untreated person infects 10 to 15 others each year. Thus, delays in diagnosis not only result in poor treatment results at the individual level, but also extended infectious periods that result in sustained transmission.
Diagnostic pitfalls
Tuberculosis (TB), which is mostly caused by Mycobacterium tuberculosis (MTB), presents significant diagnostic challenges. The gold standard for MTB diagnosis, i.e. culture, is time-consuming, costly, needs infrastructure & expertise and is prone to contamination.
Smear microscopy is a simple and affordable method for detecting AFB, but a positive result needs at least 5,000-10,000 bacilli per milliliter of sputum, accounting for low sensitivity of the test.
Such limitations make Cartridge based Nucleic Acid Amplification tests (CBNAAT), like Genexpert/ Truenat a highly reliable option. Genexpert can help identify smear-negative cases within a few hours, while also testing for resistance to Rifampicin simultaneously. Two challenges with CBNAAT are that- it detects dead bacilli also, and, that it does not detect Non tuberculous Mycobacteria.
In recent years, Interferon Gamma Release Assays (IGRA) based on T cell immunological response have been shown to be more sensitive and specific in diagnosing TB infection, with a limitation of not being able to differentiate an active case from latent TB infection. The newer QuantiFERON TB Gold plus test has an advantage over the older QuantiFERON TB Gold in being more specific. Its results are not affected by BCG vaccination of a person or even in case of infection by majority of Non tuberculous Mycobacteria, except a few. Nonetheless, this test alone cannot be used for confirming active cases.
Innovations in TB diagnostics
NGS technologies are changing TB diagnoses by allowing for complete investigation of the TB genome. This methodology detects a wide spectrum of drug resistance mutations that standard approaches may overlook. NGS also offers extensive information about the bacterial strain, which aids epidemiological research and individualised treatment recommendations. This method is sensitive and specific in determining drug resistance across panels of TB medicines concurrently.
While NGS are high end technique requiring specific infrastructure and expertise, Line probe assays (LPA- another molecular based test) are easier in comparison and used in the detection of Mycobacterium tuberculosis and drug resistance. Compared to standard culture-based drug sensitivity, it is 78.5 per cent sensitive and 100 per cent specific for identifying drug resistance.
Impact on public health and patient care
Advancements in tuberculosis diagnoses are critical to improving public health outcomes. Enhanced TB detection accuracy aids in prompt and effective treatment, transmission rate reduction, and epidemic prevention. For patients, these advancements imply more consistent test findings, better treatment options, and a higher possibility of good outcomes.
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