Diagnostic Agreement Between Intradermal Tuberculin Test and Interferon-Gamma Release Assay in Bovine Tuberculosis Surveillance in Sicily, Italy (2024–2025)

Introduction Bovine tuberculosis (bTB), caused by Mycobacterium bovis, remains endemic in Sicily, where apparent prevalence during routine screening ranges between 2% and 3%. Effective surveillance requires integrating multiple diagnostic tools; however, evidence on their concordance under operational conditions in high-prevalence settings is limited. This study assessed diagnostic agreement between the intradermal tuberculin test (IDT), the interferon-gamma release assay (IFN-γ), and post-mortem lesion detection in cattle from the province of Palermo.

Methods A retrospective observational study was conducted on 1,801 cattle from 44 herds enrolled in the national bTB eradication programme between January 2024 and December 2025. Pairwise agreement was assessed using Cohen's kappa (κ) and prevalence- and bias-adjusted kappa (PABAK), stratified by year. Non-definitive outcomes (doubtful IDT; inconclusive IFN-γ) were excluded from binary analyses. Verification bias in the slaughtered subgroup (n = 107) was evaluated by comparing test-positive proportions against the full cohort.

Results IDT–IFN-γ concordance was substantial in 2024 (κ = 0.731; 95% CI: 0.64–0.82; n = 991) and decreased to moderate in 2025 (κ = 0.453; 95% CI: 0.33–0.57; n = 572). Inconclusive IFN-γ results increased markedly from 7.1% to 19.3%. Agreement between ante-mortem tests and post-mortem lesion detection was poor across all comparisons, with kappa estimates close to zero and wide confidence intervals, consistent with severe verification bias (IDT positivity approximately six-fold higher among slaughtered animals).

Conclusions Parallel use of IDT and IFN-γ enhances herd-level detection in endemic settings. The marked increase in inconclusive IFN-γ results points to preanalytical sources of variability, blood collection and sample transport, as priority targets for procedural standardization. Post-mortem inspection is indispensable for official confirmation, but yields meaningful agreement estimates only when animals are selected for slaughter independently of their ante-mortem test result. Integrated diagnostic strategies remain critical for bTB control in persistently endemic regions.