Which tests are commonly used to diagnose latent TB infection?

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

Answer: (C)

Diagnosing latent TB infection is about detecting whether the immune system has been sensitized to TB antigens, not identifying active bacteria. The standard tests look for that immune memory.

The tuberculin skin test involves injecting a small amount of purified protein derivative under the skin and checking the reaction after 48–72 hours. A positive response indicates prior exposure to TB, though it doesn’t prove active disease. The interferon-gamma release assay uses a blood sample to measure how strongly T cells release interferon-gamma in response to TB-specific antigens. It also signals prior infection and is more specific than the skin test in people who have received the BCG vaccine.

Chest X-ray isn’t used to diagnose latent TB on its own because a normal chest image doesn’t rule out latent infection, and the test isn’t designed to detect immune sensitization. Sputum culture is aimed at active TB: it looks for live bacteria in the sputum to confirm active disease, not latent infection. Blood tests for TB antibodies aren’t reliable for LTBI or active TB, because antibodies to TB aren’t a dependable marker of infection or disease status.

So, the best approach to identify latent TB infection is through tests that measure immune sensitization to TB antigens—specifically the tuberculin skin test or interferon-gamma release assays.