Which statement about gamma-interferon release assays for diagnosing tuberculosis is true?

• A. Can distinguish between current and past infections
• B. Is based on release of gamma interferon by T cells
• C. Produces a false-positive result in patients who have received the PPD skin test
• D. Produces a false-positive result in patients who have received the tuberculosis vaccine

Answer: (B)

Gamma-interferon release assays detect tuberculosis infection by measuring the interferon-γ produced by T cells when they are stimulated with TB-specific antigens. In these tests, blood is exposed to antigens such as ESAT-6 and CFP-10; if a person has been infected, memory T cells release IFN-γ, which is then measured. This mechanism is exactly why the statement is true: the assay’s basis is the release of gamma-interferon by T cells in response to TB antigens.

These tests indicate TB infection but do not distinguish active disease from latent infection. They are designed to avoid cross-reactivity with the BCG vaccine strains, so prior vaccination with TB vaccine does not generate a false-positive IGRA result. Likewise, prior PPD skin testing does not cause a false-positive IGRA result.