Serial IGRA testing and the impact of tuberculin skin testing

The interferon-gamma release assays (IGRAs) have been developed to overcome the drawbacks of the TST. But there are still many unresolved issues related to the use of IGRAs. The aims of this study were to: 1) document 'within-subject' variability (short-term reproducibility over a 3-week period), and 2) examine the effect of the TST on IGRA responses. The study found for the QFT-GIT test that serially tested IFN-gamma responses may vary in an individual by 80% from any given initial value week to week. For the T-SPOT test, serially tested IFN-gamma spot forming counts may vary in an individual by three spots from any given initial value week to week. Boosting did occur after day 3 of the TST and occurred in both the T-SPOT and the QFT-GIT assays. Boosting occurred predominantly in IGRA-positive subjects but it also occurred in IGRA-negative subjects in a smaller but significant percentage (12.5%). Dr van Zyl-Smit proposed an 'uncertainty zone' and a threshold for conversions and reversions. The manufacturer defined assay cut-off points are: >0.35 IU/ml for QFT-GIT; and spots for T-SPOT The within-subject short-term variability for the QFT test is +/-80% of IFN-gamma response and for T-SPOT it is +/-3 spots. The borderline or uncertainty zone for the QFT is 0.2-0.7 IU/mL and for the T-SPOT it is 4-8 spots (inclusive). The proposed conversion threshold for QFT is to increase from below 0.35 to above 0.7 IU/mL and for T-SPOT to increase from below 6 to above 9 spots (inclusive). Dr van Zyl-Smit stressed the importance of taking within-subject variability into consideration when interpreting serial test results, particularly those close to the assay cut-off point. The TST boosts IGRA responses, and this occurs after day 3 and predominantly in IGRA-positive subjects, but there are also a significant number of IGRA-negative subjects. If the 'two-step strategy' is to be used, IGRAs should ideally be performed within 3 days after the TST to avoid confounding by the TST. Further large-scale prospective studies in high- and low burden-settings are required to validate these findings.