A healthcare worker has a history of BCG vaccination at birth. An IGRA (QuantiFERON-TB Gold Plus) test shows IFN-γ release of 0.8 IU/mL in TB1 (ESAT-6/CFP-10) and 0.3 IU/mL in the Nil control. The result is interpreted as POSITIVE. Which statement best explains why IGRA is preferred over TST in BCG-vaccinated individuals?
- A IGRA measures CD4 T-cell counts directly, unaffected by vaccination
- B IGRA has 100% sensitivity for active TB while TST has only 60%
- C BCG vaccination causes false-negative TST due to immune tolerance
- D ESAT-6 and CFP-10 antigens are not encoded by BCG vaccine strains; therefore BCG vaccination does not cause a false-positive IGRA result ✓
Explanation
The IGRA (QuantiFERON-TB Gold Plus and T-SPOT.TB) uses ESAT-6 (early secretory antigenic target-6) and CFP-10 (culture filtrate protein-10) as stimulating antigens. These proteins are encoded by the RD1 (region of difference 1) genomic segment, which is present in M. tuberculosis complex but deleted from BCG vaccine strains and most non-tuberculous mycobacteria. Therefore, prior BCG vaccination does not generate an IGRA-positive response, eliminating the major source of false positives seen with the tuberculin skin test (PPD contains shared antigens with BCG). IGRA detects latent TB infection and cannot diagnose active vs. latent TB.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.