Which test is preferred to assess latent TB infection in someone who has received BCG vaccination?

• A. Interferon-gamma release assay (IGRA).
• B. Purified protein derivative (PPD) tuberculin skin test.
• C. Chest x-ray.
• D. Sputum acid-fast smear.

Answer: (A)

In someone who has received BCG, testing for latent TB infection relies on a test that isn’t affected by vaccination. The interferon-gamma release assay uses TB-specific antigens that are not present in the BCG vaccine, so it’s much more specific in this group and fewer false positives occur compared with the PPD tuberculin skin test. A positive IGRA indicates latent TB infection and prompts further evaluation and possible treatment, depending on risk factors. Chest X-ray is used to look for signs of active TB disease rather than latent infection, and a sputum smear detects bacteria in active pulmonary TB, not latent infection.