Quantiferon-TB Gold In-Tube Tests In Hospital Health Care Workers: A Five Years Experience

Introduction. Interferon Gamma Release Assays (IGRAs) are being largely used worldwide for the screening of tuberculosis infection among subjects likely to undergo multiple testing, such as health care workers (HCW). In theory, IGRAs should overcome the risk of false positive results due to the boosting effect, at difference with the in vivo tuberculin skin test. Evaluation of IGRA results in HCW in a non-experimental setting over several years may provide information on the real-life performance of these tests in daily practice. We reviewed the results of QuantiFERON-TB In-Tube (QTF-IT) tests performed on HCW over nearly five years, with particular focus on repeated tests. Methods: We extracted the anonymised electronic records of all consecutive QFT-IT performed on HCW between May 2006 and December 2010. All tests were done at the Laboratory of Microbiology and Virology of University Hospital of Modena (Italy). Results: A total of 1,531 tests were performed in 1,189 individuals (mean age ± SD: 37±10 years; 29% male). In 226 subjects (37±9 years; 31% male) QFT-IT was repeated at least once. Among subjects who underwent single testing (n=963, 81%), 85% were negative and 14% positive, as compared to the results at first test among subjects with repeated tests (69% negative, 27% positive; p<0.0001). In the majority of cases (84%) a second QFT-IT provided a concordant valid result. Reversion (from positive to negative) occurred more frequently than conversion (from negative to positive) (respectively, 10% vs. 4% of repeated tests). Rate of indeterminate results was extremely low, 0.4% in subjects with single testing and 1.8% at first test in subjects with multiple tests. At second testing, indeterminate QFT-IT results at first testing became negative in all but one case, which remained indeterminate. Conclusions: In this non-experimental routine setting of tuberculosis infection screening in HCW, subjects who underwent repeated tests were more likely to have a positive QFT-IT at first testing, as compared to subjects with single testing. However, a repeated QFT-IT confirmed previous results in almost all cases. Reversions occurred more often than conversions. Rate of indeterminate QFT-IT results was extremely low, thus indicating a very good technical performance of this test in HCW in a low TB prevalence area.