A 32-year-old man from rural Maharashtra presents with a 3-month history of productive cough, fever, and night sweats. Chest X-ray shows cavitary lesions in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli. Which investigation is most appropriate to confirm drug susceptibility and guide treatment in this patient?

Explanation

## Investigation of Choice for TB Drug Susceptibility **Key Point:** Culture on solid media (Löwenstein-Jensen or Ogawa) remains the gold standard for Mycobacterium tuberculosis isolation and is essential for drug susceptibility testing (DST), which guides treatment decisions in smear-positive TB. **High-Yield:** While sputum smear microscopy confirms TB diagnosis (positive in this case), it does NOT provide information about drug resistance. DST is mandatory for: - Detecting multi-drug resistant TB (MDR-TB: resistance to isoniazid + rifampicin) - Detecting extensively drug-resistant TB (XDR-TB) - Guiding second-line therapy ### Why Culture + DST? | Investigation | Purpose | When Used | |---|---|---| | Sputum smear microscopy | Diagnosis, infectivity assessment | Initial screening | | Sputum culture + DST | Confirm diagnosis, assess drug resistance | All TB cases (mandatory in India) | | GeneXpert MTB/RIF | Rapid TB + RIF resistance detection | Presumptive TB, high-risk groups | | HPLC/chromatography | Detect drug metabolites | Research, not routine | **Clinical Pearl:** In India's NTEP (National TB Elimination Programme), culture + DST is recommended for all TB patients at baseline to detect resistance early and prevent treatment failure. **Mnemonic — TB Diagnostic Ladder:** **SCAB** - **S**mear microscopy → diagnosis - **C**ulture + DST → drug resistance - **A**ssay (GeneXpert) → rapid molecular confirmation - **B**ronchoscopy → only if diagnosis unclear [cite:Robbins 10e Ch 8]