Clinical meaning
M. tuberculosis is an acid-fast aerobic bacillus transmitted via airborne droplet nuclei (1-5 micrometres). After inhalation, bacilli are phagocytosed by alveolar macrophages. The organism survives intracellularly by inhibiting phagosome-lysosome fusion. Cell-mediated immunity (CD4+ T cells and activated macrophages) develops over 2-12 weeks, forming granulomas that contain but rarely eliminate the organism (latent TB infection, LTBI). Approximately 5-10% of immunocompetent individuals with LTBI will progress to active TB disease in their lifetime, with half occurring within 2 years of infection. Reactivation risk increases dramatically with HIV (7-10% per year), TNF-alpha inhibitors, organ transplant, and other immunosuppressive states. Active TB presents as pulmonary (85% of cases) or extrapulmonary disease (lymph node, pleural, bone, CNS). The clinician must understand the difference between LTBI (positive TST/IGRA, no symptoms, non-infectious) and active TB (symptoms, potentially infectious, requires multi-drug therapy and public health notification).
Diagnosis & workup
Diagnostics & workup: - Tuberculin skin test (TST/Mantoux): >= 5mm positive in HIV+, contacts, CXR changes; >= 10mm in other risk groups - Interferon-gamma release assay (IGRA): QuantiFERON-TB Gold or T-SPOT (preferred in BCG-vaccinated) - Chest radiograph: upper lobe infiltrates, cavitation, hilar lymphadenopathy - Sputum AFB smear x3 (early morning specimens on consecutive days) - Sputum mycobacterial culture (gold standard, takes 2-8 weeks) - GeneXpert MTB/RIF (rapid PCR for TB DNA and rifampin resistance in 2 hours)