Clinical meaning
M. tuberculosis is an aerobic, slow-growing acid-fast bacillus that establishes infection primarily in the lungs. After inhalation, bacilli are phagocytosed by alveolar macrophages but resist intracellular killing by inhibiting phagosome-lysosome fusion. Cell-mediated immunity (Th1 response) is activated after 2-8 weeks, producing granulomas that contain but do not eliminate infection. The Ghon complex (primary lung focus + draining lymph node) represents contained primary infection. Latent TB infection (LTBI) harbors dormant bacilli within granulomas — 5-10% lifetime risk of reactivation, increased to 5-15% per year in HIV with CD4 < 200. Active TB occurs when immune surveillance fails. Cavitary disease develops in the upper lobes (highest oxygen tension promotes bacillary growth), producing cough with hemoptysis, weight loss, night sweats, and high bacillary load (infectious). Drug resistance arises from spontaneous chromosomal mutations; combination therapy with RIPE prevents selection of resistant mutants.
Diagnosis & workup
Diagnostics & workup: - CXR: upper lobe infiltrates, cavitation (reactivation); hilar lymphadenopathy, middle/lower lobe infiltrates (primary); miliary pattern (disseminated) - Sputum AFB smear × 3 (induced if unable to produce): sensitivity 50-80% for pulmonary TB - Sputum culture on Lowenstein-Jensen or BACTEC MGIT (gold standard): takes 2-8 weeks but provides susceptibility testing - GeneXpert MTB/RIF: rapid molecular test (2 hours) detecting M. tuberculosis AND rifampin resistance — sensitivity > 95% for smear-positive, 67% for smear-negative - TST (Mantoux): >= 5 mm positive in HIV, close contacts, CXR changes, immunosuppressed; >= 10 mm for high-risk groups; >= 15 mm for low-risk - IGRA (QuantiFERON-TB Gold, T-SPOT): interferon-gamma release assay — no false positives from BCG vaccination; preferred for BCG-vaccinated populations - Drug susceptibility testing (DST): essential for all culture-positive cases; rapid molecular DST for rifampin and isoniazid resistance - HIV testing: mandatory for all TB patients (co-infection changes management significantly)