## Distinguishing Feature: Transmissibility **Key Point:** The critical epidemiological distinction between pulmonary TB (PTB) and extrapulmonary TB (EPTB) is the presence of acid-fast bacilli (AFB) in respiratory secretions, which determines transmissibility and public health significance. ### Comparison Table | Feature | Pulmonary TB | Extrapulmonary TB | | --- | --- | --- | | **AFB in sputum/respiratory secretions** | Yes (infectious) | No (non-infectious) | | **Transmissibility** | High (airborne spread) | Minimal to none | | **Contagiousness period** | Until treatment initiated | Not contagious | | **Public health priority** | Case-finding, contact tracing | Individual management | | **Mortality rate** | Lower if treated | Variable, depends on site | | **Prevalence in immunocompromised** | Both equally common | More common in severe immunosuppression | **High-Yield:** Only PTB with positive sputum smear (smear-positive) is considered infectious. EPTB cases, even with culture-positive disease, do not transmit TB through respiratory route and are not a source of community transmission. **Clinical Pearl:** A patient with TB meningitis or TB peritonitis, despite having active TB disease, is NOT infectious and does not require respiratory isolation. This distinction is crucial for infection control and epidemiological classification. **Mnemonic:** **EPTB-NI** = Extrapulmonary TB = Non-Infectious (in terms of airborne transmission). ### Public Health Implication Only smear-positive PTB cases are counted as "infectious TB" for: - Contact tracing protocols - Isolation requirements - Epidemiological surveillance - Case notification priorities [cite:Park 26e Ch 7]
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