## Investigation of Choice for Smear-Negative Suspected TB **Key Point:** Sputum culture on Löwenstein-Jensen (LJ) medium is the gold standard and investigation of choice for confirming pulmonary TB when sputum smear microscopy is negative but clinical and radiological suspicion remains high. ### Why Culture is Superior Sputum culture has the following advantages: - **Sensitivity:** 80–90% even in smear-negative cases (vs. 50–60% for smear microscopy) - **Gold standard:** Remains the reference standard for TB diagnosis - **Allows drug susceptibility testing (DST):** Essential for detecting MDR-TB and XDR-TB - **Turnaround time:** 2–8 weeks on LJ medium (faster on liquid media like MGIT) ### Role of Other Investigations | Investigation | Role | Limitation | |---|---|---| | **Sputum smear microscopy** | Rapid, cost-effective screening | Low sensitivity in smear-negative TB (30–50%) | | **HRCT chest** | Aids diagnosis, assesses extent | Not confirmatory; cannot differentiate TB from other infections | | **Bronchoscopy with BAL** | Reserved for suspected endobronchial TB or when sputum cannot be obtained | Invasive; not first-line | | **Mantoux test** | Indicates TB infection/immunity | Non-specific; positive in latent TB, BCG-vaccinated individuals, and NTM infections | **Clinical Pearl:** In India, the NTEP (National TB Elimination Programme) recommends sputum culture as the confirmatory test when smear microscopy is negative but clinical and radiological findings are suggestive. **High-Yield:** Smear-negative TB accounts for ~30–40% of pulmonary TB cases in India. Culture confirmation is mandatory before starting anti-TB therapy in these patients to avoid unnecessary treatment of non-TB conditions. 
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