title: 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.
abstract: 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 (Se(r)) 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) (Sp(r)) 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 Se(r) of 93% [84-98] and Sp(r) of 71.8% [67.9-75.6]. Analysis of positive results with PPD and R (PPDUR), ideal for low-risk contexts, provided a test Sp(r) of 94.3% [92.0-96.1] and Se(r) 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.
authors.affiliation: Regional Analysis and Research Laboratory of Dordogne (LDAR24), 161 Av. Churchill, 24660 Coulounieix-Chamiers, France.
authors.forename: Sandy
authors.initials: S
authors.lastname: Faye
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Moyen
authors.forename: Hélène
authors.initials: H
authors.lastname: Gares
authors.forename: Jean-Jacques
authors.initials: JJ
authors.lastname: Benet
authors.forename: Bruno
authors.initials: B
authors.lastname: Garin-Bastuji
authors.forename: María-Laura
authors.initials: ML
authors.lastname: Boschiroli
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000942:Antigens, Bacterial / Q000032:analysis; D001426:Bacterial Proteins / Q000032:analysis; D002417:Cattle / Q000276:immunology / Q000382:microbiology*; D005602:France / Q000453:epidemiology; D007371:Interferon-gamma / Q000032:analysis*; D025202:Molecular Diagnostic Techniques / Q000379:methods / Q000662:veterinary; D008969:Molecular Sequence Data; D015995:Prevalence; D012680:Sensitivity and Specificity; D012882:Skin Tests / Q000379:methods / Q000662:veterinary*; D014380:Tuberculosis, Bovine / Q000175:diagnosis* / Q000453:epidemiology / Q000276:immunology
chemical_list: D000942:Antigens, Bacterial; D001426:Bacterial Proteins; C553218:CFP-10 protein, Mycobacterium bovis; D007371:Interferon-gamma