Which statement about TB screening tests is true?

Which statement about TB screening tests is true?

• A. IGRA results are not affected by prior BCG vaccination
• B. IGRA can distinguish latent from active TB
• C. TST is unaffected by BCG vaccination
• D. Both tests are equally reliable in BCG-vaccinated individuals

Answer: (A)

Interferon-gamma release assays (IGRAs) detect TB infection using antigens (ESAT-6 and CFP-10) that are not present in most BCG vaccine strains, so prior BCG vaccination does not cause a false-positive IGRA. That means IGRA results aren’t confounded by vaccination status, unlike the tuberculin skin test (TST), which uses purified protein derivative and can react due to BCG or other non-tuberculous mycobacteria. Therefore, the statement that IGRA results are not affected by prior BCG vaccination is true. It’s also important to remember that an IGRA indicates TB infection (latent or active) but does not distinguish active disease from latent infection. In vaccinated individuals, IGRA is generally more specific than the TST because of less cross-reactivity, though cost and practicality differ. The other statements are not correct: a TST can be affected by BCG, and the two tests are not equally reliable in BCG-vaccinated people.

Interferon-gamma release assays (IGRAs) detect TB infection using antigens (ESAT-6 and CFP-10) that are not present in most BCG vaccine strains, so prior BCG vaccination does not cause a false-positive IGRA. That means IGRA results aren’t confounded by vaccination status, unlike the tuberculin skin test (TST), which uses purified protein derivative and can react due to BCG or other non-tuberculous mycobacteria. Therefore, the statement that IGRA results are not affected by prior BCG vaccination is true.

It’s also important to remember that an IGRA indicates TB infection (latent or active) but does not distinguish active disease from latent infection. In vaccinated individuals, IGRA is generally more specific than the TST because of less cross-reactivity, though cost and practicality differ. The other statements are not correct: a TST can be affected by BCG, and the two tests are not equally reliable in BCG-vaccinated people.