Diagnostic Utility of New Equation for Active Tuberculosis Based on Parameters of Interferon-γ Release Assay.

Background: We derived a new equation to include interferon-γ (IFN-γ) levels in the QuantiFERON-Gold In-Tube (QFT-GIT) assay to discriminate active tuberculosis (ATB) infection from latent TB, and compared the diagnostic performance of the QFT-GIT assay and the new equation.

Methods: From January 2013 through May 2015, we retrospectively enrolled 159 and 408 patients with and without ATB, respectively, in this study. Discriminant analysis was performed to derive an equation to distinguish the ATB group from the non-ATB group.

Results: The sensitivity and specificity of the QFT-GIT assay for diagnosing ATB were 90.6% and 63.0%, respectively. The positive and negative predictive values of the QFT-GIT assay were 48.8% and 94.5%, respectively. When the optimal cutoff for the new equation [Z = (0.031 × Nil) + (0.007 × TBAg) - 0.978] was set to -0.435, the sensitivity and specificity were 84.3% and 69.1% (positive predictive value, 51.5%; negative predictive value, 91.9%), respectively.

Conclusions: The QFT-GIT assay and the equation derived from each IFN-γ could not discriminate ATB from latent TB without considering other cytokines involved in immunity against TB.