Diagnostic accuracy of ESAT-6 free IGRA compared to QuantiFERON-TB Gold In-tube.

Background: ESAT-6 is an immunodominant Mycobacterium tuberculosis (M.tb) antigen included in novel vaccines against tuberculosis (TB) as well as in IFN release assays (IGRA). Therefore, the availability of an ESAT-6 free IGRA is essential to determine M.tb infection status following vaccination with ESAT-6-containing vaccines. We aimed to qualify a recently developed ESAT-6 free IGRA and to assess its diagnostic performance in comparison to QuantiFERON-TB Gold In-Tube (QFT).

Methods: Participants with different levels of M.tb exposure and TB disease were enrolled to determine the ESAT-6 free IGRA cut-off, to test assay performance in independent cohorts compared to standard QFT and to perform a technical qualification of antigen-coated blood collection tubes.

Results: ESAT-6 free IGRA antigen recognition was evaluated in South African adolescents with positive and negative QFT status. The ESAT-6 free IGRA cut-off was established at 0.61 IU/mL, based on ROC analysis in M.tb-unexposed controls and microbiologically-confirmed pulmonary TB patients. In an independent cohort of healthy South African adolescents, levels of IFN released in QFT and ESAT-6 free IGRA were highly correlated (p<0.0001, r=0.83) and yielded comparable positivity rates: 41.5% and 43.5% respectively, with 91% concordance between the tests (kappa = 0.82, 95% CI: 0.74-0.90; McNemar's p=0.48). ESAT-6 free IGRA blood collection tubes had acceptable lot-to-lot variability, precision and stability.

Conclusions: The novel ESAT-6 free IGRA had comparable diagnostic accuracy to QFT and is suitable to be employed in clinical trials of candidate TB vaccines to assess efficacy to prevent established M.tb infection.