Paper I — 2018 July (Supplementary) (2010 Scheme) — Q13
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Paper I
2018 July (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Lab. diagnosis of pulmonary tuberculosis.
Q132 marksShort Notes
Answer
Laboratory diagnosis of pulmonary tuberculosis (caused by Mycobacterium tuberculosis) relies on microscopy, culture, molecular, and radiological methods:
Sputum smear microscopy — Ziehl-Neelsen (ZN) staining demonstrates acid-fast bacilli on at least two sputum samples; fluorescence microscopy (auramine-rhodamine) is more sensitive and faster to screen.
Culture — the gold standard for confirmation. Lowenstein-Jensen medium (egg-based, malachite-green selective) gives characteristic rough, buff-coloured “cauliflower” colonies but is slow (2–8 weeks); liquid culture systems (BACTEC MGIT) give faster results and permit drug-susceptibility testing.
Molecular methods — CBNAAT/GeneXpert MTB-RIF, a rapid PCR-based test detecting M. tuberculosis DNA and screening for rifampicin resistance within about 2 hours.
Tuberculin skin test (Mantoux) — indicates prior sensitization/exposure, not necessarily active disease; can be falsely positive after BCG vaccination.
Interferon-Gamma Release Assays (IGRAs) — more specific than TST, unaffected by BCG vaccination status.
Radiology — chest X-ray showing apical infiltrates, cavitation, or fibrosis.
Histopathology — where tissue is available, shows caseating granulomas with epithelioid cells and Langhans giant cells.
Sputum smear microscopy remains the primary rapid, low-cost tool, with CBNAAT increasingly preferred as the initial test where available, and culture reserved for confirmation and drug-susceptibility testing.
