Comparison of the change in QuantiFERON-TB Gold Plus and QuantiFERON-TB Gold In-Tube results after preventive therapy for latent tuberculosis infection

<b>Background:</b> We investigated the change in the interferon-γ (IFN-γ) level before and after treatment of latent tuberculosis infection (LTBI) using QuantiFERON-TB Gold Plus (QFT-Plus) and QuantiFERON-TB Gold In-Tube (QFT-GIT) assays. The objective was to assess whether QFT-Plus can serve as a biomarker of LTBI treatment response. <b>Methods:</b> We prospectively enrolled 44 patients whose baseline QFT-GIT and QFT-Plus showed positive results at a tertiary referral center in South Korea between March 2017 and March 2018. After LTBI treatment, both the tests were performed again. <b>Results:</b> The mean age of the subjects was 47.6 years. The baseline IFN-γ levels were comparable among the three values; the median IFN-γ level with QFT-GIT, QFT-Plus TB1, and QFT-Plus TB2 was 3.395 IU/mL (interquartile range [IQR] 1.615–7.180), 3.060 IU/mL (IQR 0.655–7.045), and 2.880 IU/mL (IQR 0.668–7.170) (P = 0.060), respectively. Subsequently, the difference between pre- and post-LTBI treatment IFN-γ levels was measured using the QFT-GIT and QFT-Plus assays. No significant difference was noted among the three values: the median difference in IFN-γ value with QFT-GIT, QFT-Plus TB1, and QFT-Plus TB2 was 0.211 IU/mL (IQR -0.337–3.347), 0.025 IU/mL (IQR -0.338–1.368), and 0.180 IU/mL (IQR -0.490–2.278), respectively (P = 0.401). <b>Conclusions:</b> The change in the IFN-γ level before and after LTBI treatment measured using QFT-Plus assay showed a similar trend to that of QFT-GIT assay. Considering that QFT-GIT assay is not a useful biomarker of LTBI treatment response, QFT-Plus may also not be useful for this purpose.