A 35-year-old male with newly diagnosed smear-positive pulmonary TB and a 28-year-old female household contact (asymptomatic, tuberculin skin test positive, chest X-ray normal) present to the TB clinic. Which epidemiological finding best distinguishes active TB disease from latent TB infection in this scenario?

## Active TB vs. Latent TB Infection: The Defining Distinction **Key Point:** The fundamental epidemiological distinction between active TB disease (ATB) and latent TB infection (LTBI) is the presence of clinical symptoms AND radiological/bacteriological evidence of disease. LTBI is asymptomatic with no radiological findings. ### Comparison Table | Feature | Active TB Disease | Latent TB Infection | | --- | --- | --- | | **Clinical symptoms** | Present (cough, fever, night sweats) | Absent | | **Radiological findings** | Present (infiltrates, cavitation) | Absent (normal CXR) | | **Bacteriological evidence** | Positive sputum smear/culture | Negative sputum/culture | | **TST/IGRA result** | Positive | Positive | | **Infectiousness** | Yes (if smear-positive) | No | | **Risk of progression to ATB** | Already active | 5–10% lifetime risk | | **Treatment approach** | Anti-TB drugs (4 drugs × 2 months) | Preventive therapy (INH × 6 months) | **High-Yield:** TST positivity ALONE does not distinguish ATB from LTBI. A positive TST indicates TB infection (either active or latent) but requires clinical and radiological correlation to diagnose active disease. **Clinical Pearl:** The female household contact in this case has LTBI (TST+, CXR normal, asymptomatic). She requires isoniazid preventive therapy (IPT) to reduce her 5–10% lifetime risk of progression to active TB. The index case (smear-positive PTB) requires standard anti-TB chemotherapy. **Mnemonic:** **SCAR** = Symptoms, Culture/Smear, Abnormal radiology, Risk assessment = Active TB Disease. ### Epidemiological Significance ```mermaid flowchart TD A[TB Infection Suspected]:::outcome --> B{TST or IGRA Positive?}:::decision B -->|No| C[No TB Infection]:::outcome B -->|Yes| D{Clinical Symptoms?}:::decision D -->|Yes| E{CXR Abnormal?}:::decision E -->|Yes| F[Active TB Disease]:::action E -->|No| G{Sputum Smear/Culture?}:::decision G -->|Positive| F G -->|Negative| H[Latent TB Infection]:::outcome D -->|No| I{CXR Normal?}:::decision I -->|Yes| H I -->|No| J{Sputum Positive?}:::decision J -->|Yes| F J -->|No| H ``` **Diagnostic Algorithm:** 1. TST/IGRA positivity = TB infection (active or latent) 2. Add clinical symptoms + radiological findings → Active TB 3. Absence of symptoms + normal CXR + negative sputum → LTBI [cite:Park 26e Ch 7; WHO TB Guidelines 2023]