QuantiFERON-TB Gold Plus (QFT-Plus) IGRA measures the release of IFN-γ from CD4 T-cells stimulated by TB-specific antigens. Which antigens are used, and why are they preferred over tuberculin (PPD) for LTBI screening?
- A ESAT-6 and CFP-10 — absent from BCG vaccine strains and most non-tuberculous mycobacteria, giving higher specificity than TST ✓
- B MPT64 and MPB70 — present in all mycobacteria including BCG, used to confirm any mycobacterial exposure
- C Purified protein derivative (PPD) — same antigens as tuberculin, but measured in vitro
- D PGL-1 antigen — highly specific for M. tuberculosis complex only
Correct answer: A. ESAT-6 and CFP-10 — absent from BCG vaccine strains and most non-tuberculous mycobacteria, giving higher specificity than TST
Explanation
IGRA assays (QFT-Plus, T-SPOT.TB) use ESAT-6 (early secreted antigenic target-6) and CFP-10 (culture filtrate protein-10) encoded by the RD1 genomic region, which is deleted from BCG vaccine strains and absent in most non-tuberculous mycobacteria (NTM). This gives IGRA approximately 95–99% specificity compared to ~60–70% for TST/Mantoux in BCG-vaccinated populations. QFT-Plus adds TB2 antigen tube to also stimulate CD8 T-cells. PGL-1 is a leprosy antigen.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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