Concordance and Interferon-gamma Response Variability between TB1 and TB2 in the QuantiFERON-TB Gold Plus Assay: Insights Across Clinical Subgroups

Background The QuantiFERON-TB Gold Plus (QFT-Plus) assay, widely used for latent tuberculosis infection (LTBI) screening, includes two antigen tubes: TB1, which stimulates T-helper cells expressing CD4 (CD4⁺ T cells), and TB2, which additionally stimulates cytotoxic T cells expressing CD8 (CD8⁺ T cells). However, the added diagnostic value of CD8⁺ stimulation in TB2 remains uncertain. This study aimed to evaluate the diagnostic agreement between TB1 and TB2 responses in the QFT-Plus assay and assess whether TB2 provides a significant incremental benefit over TB1 in detecting tuberculosis infection across demographic and clinical subgroups. Methods This was a retrospective study that included individuals aged ≥14 years who underwent QFT-Plus testing. Interferon-gamma (IFN-γ) responses in TB1 and TB2 tubes were compared using the Wilcoxon signed-rank test. Qualitative concordance was assessed using Cohen's kappa coefficient. Subgroup analyses were stratified by age, sex, diabetes, and immunosuppressive therapy. Results Among 761 participants, median IFN-γ responses were slightly higher in TB2 than TB1 (0.21 vs. 0.19 IU/mL; median delta 0.02 IU/mL; P = 0.0002). This difference was consistent but small across most subgroups. Overall concordance between TB1 and TB2 qualitative results was 94.3% (Cohen's kappa = 0.868). Agreement remained strong across sex, age, and diabetes groups, though it was lower among users of tumor necrosis factor inhibitors (kappa = 0.582). No subgroup demonstrated a clinically significant added benefit of TB2 over TB1. Conclusion TB2 elicited slightly higher IFN-γ responses than TB1, but the small delta values and high concordance suggest limited additional diagnostic value in most populations.