Which test can screen for latent TB infection and may be preferred when BCG vaccination has been given?

Which test can screen for latent TB infection and may be preferred when BCG vaccination has been given?

• A. Mantoux tuberculin skin test (TST) or IGRA
• B. Chest X-ray
• C. Sputum culture
• D. PCR for TB

Answer: (A)

Screening for latent TB infection relies on tests that detect the immune system’s memory of TB rather than imaging or detecting active bacteria. The Mantoux tuberculin skin test (TST) and interferon-gamma release assays (IGRA) are used for this purpose. The TST involves injecting a small amount of TB protein under the skin and reading the induration after 48–72 hours, but prior BCG vaccination and exposure to non-tuberculous mycobacteria can cause false positives, reducing specificity. IGRA uses TB-specific antigens not found in BCG strains, so it is more specific in people who have received the BCG vaccine. Because of this, IGRA may be preferred when BCG vaccination has been given, though either test can screen for latent infection. Chest X-ray, sputum culture, and PCR are tools for diagnosing active TB, not latent infection; they help confirm disease when symptoms exist or to identify contagious cases, not for screening asymptomatic latent infection.

Screening for latent TB infection relies on tests that detect the immune system’s memory of TB rather than imaging or detecting active bacteria. The Mantoux tuberculin skin test (TST) and interferon-gamma release assays (IGRA) are used for this purpose. The TST involves injecting a small amount of TB protein under the skin and reading the induration after 48–72 hours, but prior BCG vaccination and exposure to non-tuberculous mycobacteria can cause false positives, reducing specificity. IGRA uses TB-specific antigens not found in BCG strains, so it is more specific in people who have received the BCG vaccine. Because of this, IGRA may be preferred when BCG vaccination has been given, though either test can screen for latent infection. Chest X-ray, sputum culture, and PCR are tools for diagnosing active TB, not latent infection; they help confirm disease when symptoms exist or to identify contagious cases, not for screening asymptomatic latent infection.