Variation in interferon-γ production between QFT-Plus and QFT-GIT assays in TB contact investigation

Background Individuals with recent contact with active tuberculosis (TB) patients were screened for latent tuberculosis infection (LTBI) considering their greater relative risk for developing active TB. QuantiFERON®-TB Gold Plus (QFT-Plus) assay offers two TB-specific antigen tubes (TB1 and TB2). TB2 is designed to elicit both CD4 and CD8 T-cell responses. This mechanism could lead to increased sensitivity as compared to the QuantiFERON®-TB Gold In-Tube (QFT-GIT) assay. Our objective was to compare the LTBI diagnostic capability of QFT-Plus with that of QFT-GIT. Methods A total of 412 TB contacts (median age 44 years) were prospectively enrolled. We conducted both QFT-Plus and QFT-GIT assays concurrently. We also analyzed production of interferon-γ (IFN-γ) in response to TB-specific antigens. Results The positivity rates in QFT-Plus and QFT-GIT were 7.5% (95% confidence interval (CI), 5.4-10.5) and 5.8% (CI, 3.9-8.5), respectively, showing a significant difference (P [QFT-Plus] /IFN-γ [QFT-GIT] was 1.41 (1.00-2.00). QFT-Plus produced 1.4 times IFN-γ. Conclusions QFT-Plus revealed significantly higher positivity rate compared with QFT-GIT, which might be attributed to increased IFN-γ production. The cut-off in both QFT-Plus and QFT-GIT was 0.35 IU/mL. These effects might resulted in the higher positivity rate in QFT-Plus.