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, increasingly preferred for programmatic use.
Culture — the gold standard for confirmation. Lowenstein-Jensen (LJ) 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 (1–2 weeks) and permit 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 within about 2 hours; line-probe assays for rapid drug-susceptibility testing.
Tuberculin skin test (Mantoux) — indicates prior sensitization/exposure, does not distinguish latent infection from active disease, and 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; CT is more sensitive for subtle changes.
Histopathology — where tissue is available, shows caseating granulomas with epithelioid cells and Langhans giant cells.
Practical approach: sputum smear microscopy remains the primary rapid, low-cost tool for diagnosis and infectivity assessment, with CBNAAT increasingly used as the preferred initial test where available, and culture reserved for confirmation, drug-susceptibility testing, and smear-negative but clinically suspicious cases.
