A 4-year-old child in Delhi is found to have tuberculosis on contact tracing. Mantoux test is 12 mm induration. Chest X-ray is normal. Which feature best distinguishes latent tuberculosis infection (LTBI) from active TB disease in this age group?

Explanation

## Discriminating Feature: Clinical and Radiological Silence in LTBI ### LTBI vs. Active TB Disease in Children **Key Point:** Latent tuberculosis infection (LTBI) is defined as the presence of **Mycobacterium tuberculosis infection without active disease**—characterized by positive immunological test (TST or IGRA) but **absence of clinical symptoms and radiological evidence** of TB. ### Comparison Table | Feature | LTBI | Active TB Disease | | --- | --- | --- | | **TST/IGRA** | Positive | Positive | | **Clinical symptoms** | **Absent** | Present (cough, fever, weight loss, night sweats) | | **Chest X-ray** | **Normal** | Abnormal (lymphadenopathy, consolidation, cavitation) | | **Bacteriological evidence** | Absent | Present (sputum/gastric aspirate smear/culture positive) | | **Infectiousness** | Non-infectious | Infectious (if pulmonary/laryngeal) | | **Risk of progression** | ~5–10% lifetime | Already progressed | | **Treatment indication** | Preventive therapy (LTBI treatment) | Anti-TB chemotherapy (4–6 drugs) | **High-Yield:** The **single best discriminator** between LTBI and active TB is the **combination of positive immunological test + absence of clinical symptoms + normal chest X-ray**. A normal CXR in a TST-positive child rules out active TB disease in the vast majority of cases. **Mnemonic:** **SILENT TB** = **S**ymptoms absent, **I**mmunological test positive, **L**ung imaging normal, **E**vidence of infection (TST/IGRA), **N**o radiological findings, **T**reatment preventive, **B**acilli not detected. **Clinical Pearl:** In contact-traced children (especially household contacts of smear-positive TB), LTBI is common. These children require **isoniazid preventive therapy (IPT)** for 6 months (or 3 months of rifampicin) to prevent progression to active disease. The absence of symptoms and normal CXR is reassuring but does not exclude the need for preventive therapy. **Warning:** Do not assume that a positive TST alone means active TB—this is a common trap. A positive TST + normal CXR + no symptoms = LTBI, not active disease. Conversely, a negative TST does not rule out active TB in severely immunocompromised children or those with recent TB infection (window period). [cite:Park 26e Ch 8; WHO Guidelines on TB in Children 2022]