A 35-year-old woman from Delhi presents with a 3-month history of persistent cough and low-grade fever. Chest X-ray shows a cavitary lesion in the right upper lobe. Sputum smear microscopy for acid-fast bacilli (AFB) is negative on two occasions. Given the high pre-test probability of tuberculosis in this clinical scenario, which investigation should be performed next to confirm the diagnosis with the highest sensitivity?

Explanation

## Clinical Context This patient has a high pre-test probability of pulmonary tuberculosis (cavitary lesion on CXR, constitutional symptoms, endemic region). Sputum smear microscopy is negative, which occurs in 10–15% of culture-positive cases (smear-negative TB). ## Why GeneXpert MTB/RIF Is Correct **Key Point:** GeneXpert MTB/RIF is a WHO-endorsed, rapid nucleic acid amplification test (NAAT) with sensitivity of 95–98% for pulmonary TB, even in smear-negative cases. It detects *Mycobacterium tuberculosis* DNA and rifampicin resistance simultaneously. **High-Yield:** GeneXpert MTB/RIF is the investigation of choice for: - Smear-negative TB (sensitivity ~90% in this subgroup) - Rapid confirmation (result in 2 hours) - Simultaneous rifampicin resistance detection - High pre-test probability scenarios **Clinical Pearl:** In a high pre-test probability case with negative smear, GeneXpert has a high positive predictive value (PPV ~95–99%) and can guide treatment initiation without waiting for culture confirmation. ## Comparison Table: Investigations for Smear-Negative TB | Investigation | Sensitivity | Specificity | Time to Result | Detects RIF-R | Cost | | --- | --- | --- | --- | --- | --- | | **GeneXpert MTB/RIF** | 95–98% | 98–99% | 2 hours | Yes | Moderate | | Sputum culture (LJ) | 95–98% | 100% | 2–8 weeks | No | Low | | Repeat AFB smear | 60–70% | 99% | 1 day | No | Very low | | TST | 70–80% | 90% | 48–72 hours | No | Low | **Mnemonic:** **NAAT** = Nucleic Acid Amplification Test — fastest, most sensitive for smear-negative TB. ## Why Not the Other Options? While sputum culture is the gold standard for diagnosis and drug susceptibility testing, it takes 2–8 weeks. In a high pre-test probability case, GeneXpert provides rapid confirmation and guides early treatment initiation. Culture should be sent *alongside* GeneXpert for drug susceptibility testing but is not the next investigation of choice in this scenario.