Paper I — 2017 August (Supplementary) (2010 Scheme) — Q2
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Paper I
2017 August (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Discuss the laboratory diagnosis of pulmonary tuberculosis.
Q25 marksShort Essays
Answer
Laboratory diagnosis of pulmonary tuberculosis (caused by Mycobacterium tuberculosis) relies on a combination of microscopy, culture, molecular, and radiological methods:
Sputum smear microscopy — Ziehl-Neelsen (ZN) staining demonstrates acid-fast bacilli (AFB) on at least two sputum samples (spot and early-morning); the organism’s mycolic-acid-rich cell wall resists decolorization by acid-alcohol after carbol fuchsin staining, appearing as pink/red bacilli against a blue background. Fluorescence microscopy (auramine-rhodamine stain) is more sensitive and faster to screen, and is increasingly preferred for high-throughput programmatic use.
Culture — the gold standard for confirmation. Lowenstein-Jensen (LJ) medium (egg-based, selective with malachite green) gives characteristic rough, buff-coloured “cauliflower” colonies but is slow (2–8 weeks). Liquid culture systems (BACTEC MGIT) give faster results (1–2 weeks) and also allow drug-susceptibility testing.
Molecular methods — CBNAAT/GeneXpert MTB-RIF, a rapid, automated PCR-based test that detects M. tuberculosis DNA and simultaneously screens for rifampicin resistance (a surrogate marker for multidrug resistance) within about 2 hours; line-probe assays for rapid drug-susceptibility testing.
Tuberculin skin test (Mantoux) — intradermal injection of purified protein derivative (PPD); induration measured at 48–72 hours indicates prior sensitization/exposure, but does not distinguish latent infection from active disease, and can be falsely positive after BCG vaccination.
Interferon-Gamma Release Assays (IGRAs) — measure IFN-γ release by sensitized T cells on exposure to M. tuberculosis-specific antigens; more specific than TST, as they are not affected by prior BCG vaccination.
Radiology — chest X-ray typically shows upper-lobe infiltrates, cavitation, or fibrosis in post-primary disease; CT is more sensitive for subtle changes.
Histopathology — where tissue is available (e.g., lymph node biopsy), shows caseating granulomas with epithelioid cells and Langhans giant cells.
Practical approach: sputum smear microscopy remains the primary tool for rapid, low-cost diagnosis and infectivity assessment in most programmatic settings, with CBNAAT increasingly used as the preferred initial test where available (WHO-recommended), and culture reserved for confirmation, drug-susceptibility testing, and cases where other tests are negative but clinical suspicion remains high.
