Serial testing with an interferon-γ release assay in German healthcare workers

AIM Data concerning conversion and reversion rates in the serial testing of healthcare workers (HCWs) is rare. So far, there is no consensus on how to define and interpret interferon-gamma release assays (IGRA) conversions and reversions, or how to deal with such results. We analysed conversion and reversion rates in the serial testing of HCWs using an IGRA. METHODS The study population comprises 287 HCWs, who participated in routine occupational safety and health screening for latent tuberculosis infection (LTBI) with the QuantiFERON-TB(®) Gold In-Tube assay (QFT). Four different definitions for conversion and reversion were applied: 1) transgression or regression above/below the cut-off; 2) increase from <0.2 to >0.7 IU/ml or decrease from >0.7 to <0.2 IU/ml; 3) transgression or regression above/below the cut-off plus change of ≥0.50 IU/ml; and 4) transgression or regression above/below the cut-off plus change of ≥0.70 IU/ml. RESULTS The highest conversion and reversion rates of 6.1% (95% CI 3.5 to 9.9) and 32.6% (95% CI 19.1 to 48.5) respectively were observed with the least stringent definition of negative to positive. The most stringent definition of an increase of ≥0.7 IU/ml above the cut-point produced the lowest conversion rate of 2.5% (95% CI 0.9 to 5.3). Using an uncertainty zone from 0.2 to 0.7 IU/ml gave low conversion (2.6%) and reversion rates (15.4%). CONCLUSION Our data confirmed the findings of previous studies that suggest that a simplistic dichotomous negative to positive definition of the IGRA might be deceptive because of the high number of spontaneous conversions and reversions. Therefore using an uncertainty zone around the cut-point (e.g. 0.2 to 0.7 IU/ml) could improve the discrimination between unspecific variation around the diagnostic cut-off and true conversion or reversion.