Preliminary data on precision of QuantiFERON-TB Plus performance

From the authors:

We agree with D. Gallagher and colleagues on the need to corroborate our recently published preliminary findings with larger and well-designed studies targeting different risk groups and performed in settings with low and high burdens of tuberculosis (TB). Indeed, we do hope that our first assessment of the QuantiFERON-TB Plus (QFT-Plus) (QIAGEN GmbH, Hilden, Germany) performance characteristics, together with the data presented by D. Gallagher and colleagues, may constitute the first body of evidence necessary to guide clinical and programmatic use of QFT-Plus within the context of TB infection management and TB elimination strategy [1].

Including CD8⁺ T-cell stimulation in a commercial diagnostic assay is a major breakthrough, which opens the field to new research possibilities. In fact, several bodies of evidence support the involvement of CD8⁺ T-cells as a front-line defence in the response to Mycobacterium tuberculosis [2]. This includes the proposed correlation with the mycobacterial load in sputum samples [3], and the functional and phenotypic changes they undergo during different phases of the infection in HIV-uninfected and [4, 5] -infected subjects [6]. In this context, QFT-Plus, despite all limitations, becomes an interesting blueprint for exploring the use of interferon-γ release assay (IGRA)-related CD8⁺ signals in programmatic TB management, as well in the investigation of new-generation parameters for the interpretation of the test in other contexts.

The results on intersubject variability reported by D. Gallagher and colleagues are likely to be important, as IGRA reproducibility issues may limit their use in settings where repeated testing is necessary [7]. Going in the same direction, we reported, in the active TB patient group, an overall increased TB2 antigen interferon (IFN)-γ response when compared to TB1 (median (interquartile range): TB1 2.09 (0.83–6.52), TB2 2.88 (1–7.89) IU·mL⁻¹; Wilcoxon test p=0.0002) [8]. The significant increase in the IFN-γ release in the samples from active TB after addition of the novel antigens to the QFT-Plus assay responds to the need for high IFN-γ responses for a reliable analytical accuracy in the measurements and might help in interpreting mild fluctuations in IFN-γ responses observed during serial testing with the QuantiFERON Gold In-Tube (QFT-GIT) version.

Furthermore, the addition of the second antigen tube may be valuable in the interpretation of the borderline results in patients with unclear TB risk factors, especially in the low-risk population. However, functional T-cell assays are highly susceptible to the variability induced by numerous factors at multiple levels (manufacturing, pre-analytic, analytic and immunological) [9] and the dichotomous nature of IGRAs make them intrinsically prone to conversion/reversion phenomena.

Interpretation of data on IGRA reproducibility is challenged by the different methods used to assess variability. Traditionally, manufacturers report variability as the coefficient of variation, determined by dividing the pooled standard deviation by the overall mean TB response. As a consequence, persons with highly positive test results have less variability (percentage of the mean) when compared to persons with negative or borderline tests results and this may account for the discrepancies in QFT-GIT variability reported in literature by various investigators [10].

Therefore, despite the encouraging preliminary findings, the variability of QFT-Plus response still needs to be determined with large studies to establish appropriate cut-offs for conversion, in order to optimise the interpretation of test results in low-incidence settings.