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?
A. Bronchoscopy with bronchoalveolar lavage
B. Sputum culture on Löwenstein-Jensen medium followed by drug susceptibility testing
C. Repeat sputum smear microscopy with higher magnification
D. High-resolution computed tomography of the chest
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-YieldNEET PG
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?
Table
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
Smear microscopy → diagnosis
Culture + DST → drug resistance
Assay (GeneXpert) → rapid molecular confirmation
Bronchoscopy → only if diagnosis unclear
Robbins 10e Ch 8
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