A 32-year-old man from rural India presents with persistent cough for 3 weeks, fever, and night sweats. Chest X-ray shows an ill-defined opacity in the right upper lobe with a cavitary lesion. Sputum smear microscopy (Ziehl-Neelsen stain) is negative on two occasions. What is the most appropriate next investigation to confirm tuberculosis?

## Investigation of Choice for Smear-Negative TB **Key Point:** Gene Xpert MTB/RIF is the WHO-recommended first-line molecular test for diagnosis of tuberculosis, especially in smear-negative cases with clinical and radiological suspicion. ### Why Gene Xpert MTB/RIF? 1. **Sensitivity & Specificity** - Sensitivity: 95–98% for smear-positive TB; 80–90% for smear-negative pulmonary TB - Specificity: >99% for TB diagnosis - Detects rifampicin resistance simultaneously 2. **Rapid Turnaround** - Results available within 2 hours - Enables early diagnosis and treatment initiation 3. **WHO Endorsement** - Recommended as initial diagnostic test for all TB suspects (2021 WHO guidelines) - Particularly valuable in smear-negative, HIV-positive, and drug-resistant TB cases ### Comparison of Diagnostic Modalities | Investigation | Sensitivity (Smear-Neg) | Specificity | Time | Detects DR-TB | Cost | |---|---|---|---|---|---| | **Sputum Smear (ZN)** | 45–50% | >99% | 24 hrs | No | Low | | **Gene Xpert MTB/RIF** | 80–90% | >99% | 2 hrs | Yes (RIF) | Moderate | | **Sputum Culture** | 95–98% | >99% | 2–8 weeks | Yes | High | | **HRCT Chest** | N/A (imaging) | N/A | Immediate | No | High | | **Mantoux Test** | 70–80% | 60–70% | 48–72 hrs | No | Low | **Clinical Pearl:** In a smear-negative patient with strong clinical and radiological suspicion (cavitary lesion in upper lobe), Gene Xpert is the single best test to confirm TB and simultaneously assess for rifampicin resistance, guiding treatment decisions. **High-Yield:** Gene Xpert MTB/RIF has largely replaced sputum culture as the initial confirmatory test in resource-limited settings due to rapid turnaround and simultaneous detection of drug resistance. ### Algorithm for TB Diagnosis in Smear-Negative Cases ```mermaid flowchart TD A[Clinical suspicion of TB + Smear-negative]:::outcome --> B{Perform Gene Xpert MTB/RIF}:::decision B -->|Positive| C[TB confirmed]:::outcome C --> D{RIF resistance?}:::decision D -->|Yes| E[Start MDR-TB regimen]:::action D -->|No| F[Start standard TB regimen]:::action B -->|Negative| G[Consider alternative diagnosis]:::decision G -->|High clinical suspicion| H[Sputum culture or HRCT]:::action G -->|Low suspicion| I[Investigate other causes]:::action ``` [cite:WHO TB Guidelines 2021]