Determination of decisional cut-off values for the optimal diagnosis of bovine tuberculosis with a modified IFNγ assay (Bovigam®) in a low prevalence area in France

The Bovigam® gamma interferon (IFNγ) assay was used to complement official skin-test screening in a low bovine tuberculosis (bTB) prevalence region in France.The aim of our work was to determine decisional cut-off values for protein purified derivatives (PPD) and ESAT6-CFP10 antigens (R) in order to optimize the efficacy of the modified Bovigam® test, in this low-prevalence area, for optimal classification of infected or non-infected herds following positive skin tests.The sensitivity of the IFNγ assay relative to post-mortem bTB-positive animals (Ser) was studied in 60 cattle from 20 bTB-infected herds. Its absolute specificity (Sp) was studied in 492 cattle from 25 bTB-free herds from a bTB-free zone. Its operational specificity (relative to the positive skin test) (Spr) was also studied in 547 skin-test positive cattle from 172 bTB-free herds from an infected zone.Using normalized interpretations for individual (PPD or R) results, the cut-off values at 0.02 for PPD and 0.01 for R were obtained with a view to employ them in low prevalence areas with no previously observed non-specific reactions to SITT.Concerning its use after positive skin tests, cut-off values were set at 0.05 for PPD and at 0.03 for R. The choice of an interpretation method considering positive results with PPD and/or R (PPDUR), justified in a high risk context, provided a test Ser of 93% [84–98] and Spr of 71.8% [67.9–75.6]. Analysis of positive results with PPD and R (PPD∩R), ideal for low-risk contexts, provided a test Spr of 94.3% [92.0–96.1] and Ser of 77% [64–87].Thus, adapting the criteria to the region's infection status and to the conditions for its application is essential for the appropriate use of the IFNγ assay.