Letter to the Editor Diagnostic Immunology Repeatability of QuantiFERON-TB Gold In-Tube Assay Results Near Cut-Off Points

Interferon (IFN)-γ release assay (IGRA) is a functional assay used to measure T cell responses to Mycobacterium tuberculo-sis-specific antigens in whole blood or blood-derived mononu-clear cells [1, 2]. The QuantiFERON-TB Gold In-Tube (QFT-GIT; Cellestis, Carnegie, Australia) is a commercially available IGRA. Significant within-subject variability in QFT-GIT results has been reported in tuberculosis (TB) screening [3, 4]. Variability in QFT-GIT results for the retesting of the same patient sample has been observed in a low TB-incidence setting. Twenty-eight (8%) of 366 patients showed results different from those of the initial QFT-GIT, and 24 (85.7%) of these 28 patients had an initial TB response between 0.25 and 0.80 IU/mL [5]. To explore the clinical impact of QFT-GIT results with border-line TB response in an intermediate TB-incidence setting, we specifically assessed the repeatability of QFT-GIT near cut-of