## Investigation of Choice for Confirming Active Pulmonary TB ### Gold Standard for Diagnosis **Key Point:** Sputum smear microscopy (Ziehl-Neelsen or auramine-rhodamine staining) remains the gold standard and most cost-effective investigation for confirming active pulmonary tuberculosis in resource-limited settings, including India. ### Why Sputum Smear Microscopy? 1. **Diagnostic Accuracy**: Detects acid-fast bacilli (AFB) directly in sputum samples 2. **Cost-Effective**: Minimal equipment and reagent requirements 3. **Infectivity Marker**: Smear-positive patients are highly infectious and represent the priority for TB control programs 4. **Rapid Results**: Can be performed within hours 5. **WHO Recommendation**: Endorsed as the primary diagnostic tool in TB-endemic countries ### Diagnostic Criteria | Investigation | Sensitivity | Specificity | Role | |---|---|---|---| | Sputum smear microscopy | 50–80% (depends on bacillary load) | >95% | Gold standard for active TB | | Chest X-ray | 85–95% | 70–80% | Supportive; cannot diagnose alone | | Mantoux test | 70–90% | 60–70% | Indicates TB infection, not active disease | | HRCT thorax | >95% | 85–90% | High sensitivity but not routine | ### Clinical Pearl **High-Yield:** In a smear-positive patient (as indicated by cavitary disease and symptoms), sputum microscopy is both diagnostic AND epidemiologically significant—it identifies the source of transmission and guides public health interventions. ### Recommended Sputum Collection Protocol - Early morning sputum (highest bacillary yield) - Minimum 2–3 samples on separate days - At least 5 mL per sample - Processed within 2 hours of collection ### Mnemonic: AFB Detection Methods **ZN-AR**: **Z**iehl-**N**eelsen (carbol-fuchsin) and **A**uramine-**R**hodamine (fluorescent) staining are the two standard microscopy techniques; ZN is more widely used in resource-limited settings due to lower cost. [cite:Park 26e Ch 5]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.