A 28-year-old female healthcare worker in Mumbai is found to have a positive tuberculin skin test (TST: 15 mm induration) during routine screening. She is asymptomatic with normal chest X-ray. Which investigation is most appropriate to determine if she has latent TB infection (LTBI) versus active TB disease?

## Investigation of Choice for Latent TB Infection Confirmation **Key Point:** In an asymptomatic individual with positive TST and normal CXR, an Interferon-gamma release assay (IGRA) is the most appropriate next investigation to confirm TB infection (latent or active) and guide preventive therapy decisions. ### Why IGRA is Correct **High-Yield:** IGRA (QuantiFERON-Gold In-Tube, T-SPOT.TB) is superior to TST for: 1. **Confirmation of TB infection** — measures cell-mediated immune response to MTB antigens (ESAT-6, CFP-10) 2. **Reduced false positives** — not affected by BCG vaccination (unlike TST) 3. **Higher specificity** — better at distinguishing TB infection from non-tuberculous mycobacteria 4. **Single visit** — results available in 24 hours (vs. TST which requires 48–72 hours) ### Clinical Context: Asymptomatic + Normal CXR In this scenario: - **TST positive (15 mm)** = TB infection (latent or active) - **Asymptomatic** = no active TB symptoms - **Normal CXR** = no radiological evidence of active TB - **Healthcare worker** = high-risk group for TB exposure The patient likely has **latent TB infection (LTBI)**, and IGRA confirmation is needed before initiating preventive therapy (isoniazid monotherapy or shorter regimens). ### Comparison of Diagnostic Approaches | Investigation | Sensitivity | Specificity | BCG Effect | Time to Result | Clinical Use | |---|---|---|---|---|---| | **TST** | 70–80% | 60–70% | Yes (↑ false +) | 48–72 hours | Screening, but not confirmatory | | **IGRA (QFT-GIT)** | 85–95% | 95–98% | No | 24 hours | **Confirmation of TB infection** | | **Sputum smear** | 60–80% | High | N/A | 1–2 days | Active TB only (not LTBI) | | **CXR** | Varies | Varies | N/A | Minutes | Detects active TB (not LTBI) | | **CT chest** | High | High | N/A | Minutes | Active TB staging (not LTBI) | **Clinical Pearl:** In BCG-vaccinated individuals (common in India), TST may be falsely positive due to BCG-induced immunity. IGRA is unaffected by BCG and is more specific for TB infection. ### Algorithm: TST-Positive Asymptomatic Individual ```mermaid flowchart TD A[TST positive + Asymptomatic + Normal CXR]:::outcome --> B[IGRA: QuantiFERON-Gold]:::action B --> C{IGRA positive?}:::decision C -->|Yes| D[Latent TB Infection confirmed]:::outcome C -->|No| E[False-positive TST, no TB]:::outcome D --> F{Risk factors for progression?}:::decision F -->|Yes| G[Preventive therapy: INH 6 months]:::action F -->|No| H[Observation, no preventive therapy]:::action E --> I[No further TB workup needed]:::outcome ``` **Mnemonic: IGRA = Immune response Gamma-interferon Assay** — confirms TB infection independent of BCG vaccination.