Which tests are commonly used to diagnose latent TB infection?

• A. Chest X-ray only
• B. Tuberculin skin test (TST) or interferon-gamma release assay (IGRA)
• C. Sputum culture
• D. Blood tests for TB antibodies

Answer: (B)

Testing for latent TB infection relies on the immune system’s prior exposure to TB bacteria. The two most commonly used tests are the tuberculin skin test and the interferon-gamma release assay.

The tuberculin skin test involves injecting a small amount of purified protein derivative under the skin and checking the size of the induration after 48–72 hours. A positive result suggests TB infection, but it can be influenced by prior BCG vaccination or exposure to non-tuberculous mycobacteria, and immunosuppressed people may have false negatives. The interferon-gamma release assay uses a blood sample to measure how much interferon-gamma T cells release in response to TB-specific antigens that are not present in most BCG strains or many non-tuberculous mycobacteria, so it’s less affected by vaccination history and doesn’t require a return visit.

A positive result from either test indicates infection with TB bacteria but does not tell you whether the infection is latent or active. Additional evaluation, including symptom assessment and chest imaging, is needed to distinguish latent infection from active disease. Other options, like chest X-ray, sputum culture, or TB antibody tests, are not used to diagnose latent infection because they either assess for active disease (chest X-ray, sputum culture) or are not reliable/predictive (TB antibodies).