A 35-year-old man from rural Maharashtra presents with a 3-week history of productive cough, fever, and night sweats. Clinical examination reveals crackles in the right upper lobe. Chest X-ray shows a cavitary lesion in the right apical region. What is the investigation of choice to confirm the diagnosis of pulmonary tuberculosis under RNTCP guidelines?

## Investigation of Choice for Suspected Pulmonary TB under NTEP/RNTCP **Key Point:** Under current NTEP (National Tuberculosis Elimination Programme, formerly RNTCP) guidelines, **GeneXpert MTB/RIF (CBNAAT)** is the recommended first-line investigation for diagnosing pulmonary tuberculosis, replacing sputum smear microscopy as the initial test of choice. ### Rationale for GeneXpert MTB/RIF 1. **NTEP 2022 Guidelines**: The revised NTEP guidelines (aligned with WHO 2021 recommendations) mandate GeneXpert MTB/RIF (CBNAAT) as the **initial diagnostic test** for all presumptive TB cases, including pulmonary TB. 2. **Simultaneous detection**: GeneXpert detects both *Mycobacterium tuberculosis* complex AND rifampicin resistance (a proxy for MDR-TB) in a single test within ~2 hours. 3. **Superior sensitivity**: GeneXpert has sensitivity of ~88% for smear-positive and ~67–70% for smear-negative pulmonary TB, far exceeding ZN smear microscopy (~45–60% overall sensitivity). 4. **Universal DST mandate**: NTEP's "Universal Drug Susceptibility Testing" (UDST) policy requires that every TB patient have drug susceptibility testing at diagnosis — GeneXpert fulfills this requirement; smear microscopy does not. 5. **Cavitary disease context**: While cavitary lesions yield high bacillary loads (making smear-positive likely), GeneXpert is still preferred as it simultaneously rules out rifampicin resistance, which is critical for treatment decisions. ### NTEP Diagnostic Algorithm (Current) | Step | Test | Purpose | |------|------|---------| | **First-line** | GeneXpert MTB/RIF (CBNAAT) | Detect MTB + RIF resistance | | If GeneXpert unavailable | Sputum smear microscopy (ZN stain) | Detect AFB | | Smear-negative / GeneXpert-negative with high suspicion | CECT chest / culture (LJ/MGIT) | Further workup | ### Why Other Options Are Incorrect - **Option A (HRCT chest)**: Imaging is supportive, not confirmatory; not recommended as first-line diagnostic under NTEP. - **Option B (Sputum smear microscopy)**: Previously the first-line test under old RNTCP; now superseded by GeneXpert under current NTEP guidelines. Smear microscopy is used only when GeneXpert is unavailable. - **Option C (Mantoux/TST)**: Indicates TB infection (latent or active), not disease confirmation; not recommended for diagnosing active pulmonary TB under NTEP. **High-Yield:** The shift from smear microscopy to GeneXpert as first-line reflects India's commitment to Universal DST — every diagnosed TB patient must have drug susceptibility information at the time of diagnosis (NTEP Operational Guidelines 2022; WHO Consolidated Guidelines on TB, 2022). **Clinical Pearl:** GeneXpert MTB/RIF is available free of charge at all NTEP-designated microscopy centres and district hospitals across India, making it the accessible, rapid, and policy-mandated first-line test. [cite: NTEP Operational Guidelines 2022; WHO Consolidated Guidelines on Tuberculosis, Module 3: Diagnosis, 2021; Park's Textbook of Preventive and Social Medicine, 27e]