Enhancing the interferon-γ release assay through omission of nil and mitogen values

Abstract

Purpose: To address the limited utility of the interferon (IFN)-γ release assay (IGRA) caused by its variability and inconsistency.

Methods: This retrospective cohort study was based on data obtained between 2011 and 2019. QuantiFERON-TB Gold-In-Tube was used to measure IFN-γ levels in nil, tuberculosis (TB) antigen, and mitogen tubes.

Results: Of 9,378 cases, 431 had active TB. The non-TB group comprised 1,513 IGRA-positive, 7,202 IGRA-negative, and 232 IGRA-indeterminate cases. Nil-tube IFN-γ levels were significantly higher in the active TB group (median = 0.18 IU/mL; interquartile range: 0.09-0.45 IU/mL) than in the IGRA-positive non-TB (0.11 IU/mL; 0.06-0.23 IU/mL) and IGRA-negative non-TB (0.09 IU/mL; 0.05-0.15 IU/mL) groups (P < 0.0001). From receiver operating characteristic analysis, TB antigen tube IFN-γ levels had higher diagnostic utility for active TB than TB antigen minus nil values. In a logistic regression analysis, active TB was the main driver of higher nil values. In the active TB group, after reclassifying the results based on a TB antigen tube IFN-γ level of 0.48 IU/mL, 14/36 cases with negative results and 15/19 cases with indeterminate results became positive, while 1/376 cases with positive results became negative. Overall, the sensitivity for detecting active TB improved from 87.2 to 93.7%.

Conclusion: The results of our comprehensive assessment can aid in IGRA interpretation. Since nil values are governed by TB infection rather than reflecting background noise, TB antigen tube IFN-γ levels should be used without subtracting nil values. Despite indeterminate results, TB antigen tube IFN-γ levels can be informative.

Keywords: Immunological tests; Interferon-γ; Latent tuberculosis; Release assay; Tuberculosis.

Fig. 1

Nil (a), TBAg (b), and TBAg − Nil (c) IFN-γ levels in the active-TB, IGRA-positive non-TB, and IGRA-negative non-TB groups. Cases with indeterminate IGRA results were excluded. Boxes span the 25th to 75th percentiles, whiskers indicate the 10th and 90th percentiles, and bullets represent outliers. Nil nil tube, TBAg tuberculosis antigen tube, TBAg − Nil TBAg minus Nil, IFN-γ interferon-γ, TB tuberculosis, IGRA interferon-γ release assay, non-TB non-tuberculosis, IGRA + IGRA-positive, IGRA- IGRA-negative ****P < 0.0001

Fig. 2

Spearman’s rank correlation analysis of the IFN-γ levels of the Nil and TBAg in all subjects (a) and in the active TB (b), IGRA-positive non-TB (c), and IGRA-negative non-TB (d) groups. IFN-γ interferon-γ, Nil nil tube, TBAg tuberculosis antigen tube, TB tuberculosis, IGRA interferon-γ release assay, non-TB non-tuberculosis

Fig. 3

Forest plot of significant factors in the multivariate logistic regression to predict high Nil IFN-γ levels in the IGRA (QFT-GIT). Cases were divided into high- and low-IFN-γ Nil groups based on the median value (0.09 IU/mL). Cases with indeterminate results were excluded. Nil nil tube, IFN-γ interferon-γ, IGRA interferon-γ release assay, QFT-GIT QuantiFERON-TB Gold-in-Tube, TB tuberculosis

Fig. 4

Forest plot of significant factors in the multivariate logistic regression to predict indeterminate results in the IGRA (QFT-GIT). Cases were divided according to the criterion for indeterminate results (Mitogen − Nil IFN-γ level < 0.5 IU/mL). For definitions of lymphopenia and hypoalbuminemia, refer to the Methods. IGRA interferon-γ release assay, QFT-GIT QuantiFERON-TB Gold-in-Tube, Mitogen mitogen tube, Nil nil tube, IFN-γ interferon-γ