## Distinguishing PTB from EPTB Under RNTCP ### Key Diagnostic Criterion **Key Point:** The hallmark discriminator between PTB and EPTB under RNTCP is the **presence of acid-fast bacilli (AFB) on sputum smear microscopy**, which defines PTB as a bacteriologically confirmed case. ### RNTCP Case Definitions | Feature | Pulmonary TB (PTB) | Extrapulmonary TB (EPTB) | |---------|-------------------|-------------------------| | **AFB on sputum smear** | Present (smear-positive) or absent (smear-negative) | Absent in sputum; diagnosed by other means (histology, culture, imaging, clinical) | | **Diagnostic method** | Sputum microscopy (primary) | Biopsy, culture, imaging, clinical features | | **Transmissibility** | Smear-positive cases are infectious | Non-infectious | | **Reporting priority** | Mandatory notification | Mandatory but lower epidemiological impact | | **Common sites** | Lungs (apical-posterior segments) | Lymph nodes, pleura, spine, CNS, abdomen | ### Why AFB on Sputum is the Best Discriminator 1. **Bacteriological confirmation:** Only PTB can be diagnosed by sputum smear microscopy; EPTB cannot. 2. **Public health significance:** Smear-positive PTB cases are the primary source of TB transmission in the community. 3. **RNTCP classification:** RNTCP explicitly defines "Pulmonary TB" as disease of the lungs detectable by sputum examination. 4. **Operational simplicity:** Sputum microscopy is the gold standard screening tool under RNTCP in resource-limited settings. **Clinical Pearl:** A patient with TB lymphadenitis (EPTB) may have positive sputum smear if there is concurrent pulmonary involvement—in such cases, the case is classified as PTB, not EPTB. **High-Yield:** Under RNTCP, smear-positive PTB is the epidemiological unit of surveillance; EPTB cases are tracked separately and do not contribute to the TB transmission chain in the same way.
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