Identifying Past Tuberculosis Infection: Tests, Imaging & Clinical Clues

Diagnostic Approaches for Historical TB

1. Medical Records Review

• Prior TB diagnosis, treatment records (anti-TB drugs: isoniazid, rifampin, etc.).
• Positive tuberculin skin test (TST/PPD) or IGRA (QuantiFERON, T-SPOT) documentation.
• Old chest X-rays: apical fibronodular scarring, calcified granulomas, pleural thickening.

2. Current Testing for Latent TB Infection (LTBI)

• TST (PPD): Induration ≥10 mm (5 mm if high-risk). False positives from BCG vaccine; false negatives in anergy.
• IGRA (blood test): Measures IFN-γ response to TB-specific antigens (ESAT-6, CFP-10). Unaffected by BCG. Preferred for BCG-vaccinated.

3. Chest Imaging

• Chest X-ray: Look for upper lobe fibrosis, calcified nodules, pleural plaques.
• CT: More sensitive for subtle scarring, tree-in-bud, bronchiectasis from prior TB.

4. Clinical History

• Past symptoms: chronic cough, hemoptysis, night sweats, weight loss, fever.
• Known exposure: household/workplace contact with active TB.
• Risk factors: birth/travel in endemic areas, HIV, immunosuppression, homelessness, incarceration.

5. Sputum Culture (If Symptomatic)

Gold standard for active TB. Not for past infection unless reactivation suspected.

Interpretation & Next Steps

Positive IGRA/TST + normal CXR = LTBI (consider treatment). Positive + abnormal CXR = rule out active TB. Always consult a TB specialist or public health provider for context-specific interpretation.