Paper II — 2022 July (Supplementary) (2019 Scheme) — Q4
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Paper II
2022 July (Supplementary) (2019 Scheme) · 100 marks · 180 min
Question
Discuss the molecular methods available for the diagnosis of tuberculosis.
Q48 marksShort Essays
Answer
Molecular methods for the diagnosis of tuberculosis
CBNAAT / GeneXpert MTB/RIF (Cartridge-Based Nucleic Acid Amplification Test)
A real-time PCR-based, fully automated, cartridge-based assay that simultaneously detects Mycobacterium tuberculosis complex DNA and mutations in the rpoB gene associated with rifampicin resistance, within about 2 hours. Widely adopted as an initial rapid diagnostic test in India’s National TB Elimination Programme, given its high sensitivity (particularly for smear-positive disease), rapid turnaround, minimal biosafety requirements, and simultaneous rifampicin-resistance screening (a proxy marker for MDR-TB).
Truenat MTB / MTB-Rif / MTB-Plus
A portable, chip-based real-time PCR platform, usable at point-of-care/peripheral health facility level (battery-operated), detecting M. tuberculosis DNA and, in the Rif assay, rifampicin-resistance mutations — offering similar rapid, decentralized testing capability to CBNAAT with greater portability.
Line Probe Assay (LPA)
A PCR-based hybridization assay using strips with immobilized probes, capable of detecting M. tuberculosis complex and specific mutations conferring resistance to first-line drugs (isoniazid, rifampicin — via the “first-line LPA”) and second-line drugs (fluoroquinolones, injectable agents — via the “second-line LPA”), providing a more detailed drug-resistance profile than rifampicin-only tests, used particularly for diagnosing/characterizing drug-resistant TB.
Loop-Mediated Isothermal Amplification (TB-LAMP)
An isothermal nucleic acid amplification technique (does not require thermal cycling equipment), offering a simpler, lower-cost molecular alternative for TB diagnosis in resource-limited settings, with sensitivity approaching smear microscopy plus culture in some studies.
Whole Genome Sequencing (WGS) / Next-Generation Sequencing
Increasingly used in reference laboratories for comprehensive drug-resistance profiling (detecting mutations across multiple resistance-associated genes simultaneously) and for strain typing/outbreak investigation.
Advantages of molecular methods overall: rapid turnaround (hours rather than weeks, as required by conventional culture); ability to simultaneously detect drug resistance; do not require viable organisms, making them useful even after antibiotic exposure; and improved sensitivity over smear microscopy, particularly in paucibacillary disease (e.g., extrapulmonary TB, paediatric TB, HIV-associated TB).
Limitations: cannot fully replace culture, which remains necessary for comprehensive phenotypic drug-susceptibility testing (against all first- and second-line drugs) and for obtaining isolates for further characterization; molecular tests are also generally more expensive than conventional smear microscopy.
