A 28-year-old female healthcare worker from Mumbai with a history of close contact with a pulmonary TB patient 6 months ago now presents with a 2-month history of low-grade fever, mild cough, and fatigue. Tuberculin skin test (TST/Mantoux test) shows an induration of 15 mm at 48 hours. Chest X-ray is normal. Sputum smear microscopy is negative for AFB on three consecutive samples. She is otherwise immunocompetent. What is the most likely diagnosis and the recommended next step in management?

Explanation

## Latent Tuberculosis Infection (LTBI) — Diagnosis and Management ### Clinical Presentation Analysis **Key Point:** This patient has clinical and epidemiological evidence of TB exposure but lacks microbiological confirmation of active disease. | Finding | Interpretation | |---------|----------------| | TST induration 15 mm | Positive (≥5 mm in immunocompetent with TB contact) | | Sputum smear × 3 negative | Rules out smear-positive pulmonary TB | | Normal chest X-ray | No radiological evidence of active TB | | Symptoms (fever, cough, fatigue) | Non-specific; could be LTBI with mild symptoms or other etiology | | Immunocompetent status | No increased risk of TB progression | ### Diagnosis: Latent Tuberculosis Infection (LTBI) **High-Yield:** LTBI is defined as: - Positive TST or IGRA (interferon-gamma release assay) - **No clinical, radiological, or bacteriological evidence of active TB** - Normal chest X-ray - Negative sputum smear/culture In this patient: - TST is positive (15 mm induration in immunocompetent person with TB contact) - Sputum smear is negative × 3 - CXR is normal - **Diagnosis = LTBI** ### Management of LTBI **Mnemonic: LTBI Treatment Regimens — "INH for 6, RIF for 4, INH-RIF for 3"** 1. **Isoniazid (INH) monotherapy for 6 months** — First-line in most settings, including India - Dose: 5 mg/kg daily (max 300 mg) - Efficacy: ~60–90% reduction in TB progression - Requires pyridoxine (vitamin B₆) supplementation to prevent peripheral neuropathy 2. **Rifampicin (RIF) monotherapy for 4 months** — Alternative if INH intolerance 3. **INH + RIF for 3 months** — Shorter regimen, higher cost 4. **INH + RPT (rifapentine) weekly for 3 months** — Newer, shorter option (not yet standard in India) **Clinical Pearl:** The risk of TB progression in untreated LTBI is ~5–10% lifetime risk; in immunocompromised patients (HIV, TNF-α inhibitors), it is 5–10% per year. This patient is immunocompetent, so standard LTBI therapy is indicated. ### Why Other Options Are Wrong ```mermaid flowchart TD A["Patient with TB Contact"]:::outcome --> B{"TST/IGRA Positive?"}:::decision B -->|Yes| C{"Evidence of Active TB?"}:::decision C -->|No CXR, No Symptoms, Smear Negative| D["Latent TB Infection"]:::outcome C -->|Yes CXR, Symptoms, Smear Positive| E["Active TB Disease"]:::urgent D --> F["Preventive Therapy"]:::action E --> G["Anti-TB Treatment"]:::action F --> H["INH 6 months or RIF 4 months"]:::action G --> I["4-drug regimen 2 months, then 2-drug 4 months"]:::action ``` **Option 1 (correct)** — Matches the clinical diagnosis and guideline-recommended management. **Option 2** — Active TB requires sputum smear/culture positivity OR clinical + radiological evidence. This patient has negative smear × 3 and normal CXR, so active TB is ruled out. Starting four-drug therapy without microbiological confirmation would be inappropriate. **Option 3** — Non-tuberculous mycobacteria (NTM) are not indicated here. The TST is positive, which is specific for TB complex (Mycobacterium tuberculosis, M. bovis, M. africanum). NTM typically cause TST-negative disease or low-grade induration. Culture and DST are not indicated without clinical/radiological suspicion of active TB. **Option 4** — While BCG vaccination can cause TST positivity, the distinction between BCG-induced and TB-induced TST is clinically irrelevant in a **TB contact with recent exposure**. IGRA (interferon-gamma release assay) can help differentiate, but given the epidemiological context (close contact 6 months ago), TST positivity should be treated as TB infection. Additionally, BCG-induced TST typically wanes after 5–10 years; a 15 mm induration in a recent contact is more likely TB-related. ### Key Guideline References **High-Yield:** According to WHO and Indian TB guidelines (NTEP/RNTCP): - All persons with LTBI and risk factors (TB contact, immunosuppression) should receive preventive therapy - INH for 6 months is the standard regimen in India - Baseline liver function tests and monthly clinical monitoring are recommended [cite:Harrison 21e Ch 158; WHO Guidelines on Tuberculosis Preventive Therapy 2020; India NTEP Guidelines 2023]