A 38-year-old woman with no prior history of tuberculosis undergoes tuberculin skin test (TST) screening at her workplace. The TST is positive (induration ≥5 mm). The TST has a sensitivity of 85% and specificity of 90% in the general population. The prevalence of active TB in her region is 1%. Which investigation is the most appropriate next step to confirm active tuberculosis?

Explanation

## Clinical Scenario Analysis A positive TST in a low-prevalence population (1% active TB) has a low positive predictive value (PPV). The TST detects both active TB and latent TB infection, making it a screening test, not a diagnostic test. ## Calculating PPV for TST $$PPV = \frac{0.85 \times 0.01}{(0.85 \times 0.01) + (0.10 \times 0.99)} = \frac{0.0085}{0.0085 + 0.099} \approx 0.079 \text{ or } 7.9\%$$ **Key Point:** A positive TST with PPV of ~8% means 92% of positive tests are false positives (latent TB infection, not active disease). Confirmatory testing is mandatory. ## Investigation Hierarchy for TB Diagnosis ```mermaid flowchart TD A[Positive TST]:::outcome --> B{Clinical symptoms?}:::decision B -->|Yes or Suspicious CXR| C[Chest X-ray + Sputum smear]:::action B -->|No symptoms| D[IGRA or repeat TST]:::action C --> E{AFB positive?}:::decision E -->|Yes| F[Confirm TB diagnosis]:::outcome E -->|No| G[Culture, GeneXpert MTB/RIF]:::action D --> H{IGRA positive?}:::decision H -->|Yes + symptoms| I[Chest X-ray]:::action H -->|No| J[Latent TB, no active disease]:::outcome ``` ## Why Chest X-ray and Sputum Smear Microscopy? | Investigation | Sensitivity | Specificity | Purpose | Diagnostic? | |---|---|---|---|---| | **TST** | 85% | 90% | Screening | No | | **Chest X-ray** | 80–90% | 70–80% | Detect infiltrates | Presumptive | | **Sputum smear microscopy** | 50–80% | 95–99% | Confirm active TB | Yes (if AFB+) | | **GeneXpert MTB/RIF** | 98% | 99% | Confirm + detect rifampicin resistance | Yes | | **IGRA** | 90% | 96% | Differentiate latent from active | No | **High-Yield:** In a patient with a positive TST and clinical suspicion of active TB, the next investigation is **chest X-ray + sputum smear microscopy** because: 1. Chest X-ray identifies infiltrates consistent with TB 2. Sputum smear microscopy (AFB staining) directly detects *Mycobacterium tuberculosis* 3. Together they provide presumptive diagnosis and infectivity assessment 4. This is the WHO-recommended algorithm for TB diagnosis in resource-limited settings **Clinical Pearl:** TST positivity alone does NOT diagnose active TB. It indicates TB infection (active or latent). Confirmation requires evidence of disease: symptoms, imaging findings, and/or microbiological confirmation (AFB smear or culture). **Mnemonic:** **AFB-CXR-TST** — After a positive screening Test, check Sputum for AFB and get a Chest X-Ray to confirm active disease. [cite:Park 26e Ch 5]