Repeated Quantiferon-TB Gold In-Tube Tests In Routine Clinical Practice: A Five Years Experience

Introduction. Interferon Gamma Release Assays (IGRAs) are increasingly being used as diagnostic aids for Mycobacterium tuberculosis infection. Although evidence on IGRA performance in different populations is quickly accumulating, data on their use in non-experimental settings may provide information on their performance in routine clinical practice. We reviewed the results of QuantiFERON-TB In-Tube (QTF-IT) tests at our hospital since its introduction in diagnostic routine, with particular focus on repeated tests. Methods: We extracted the anonymised electronic records of all consecutive QFT-IT performed at the Laboratory of Microbiology and Virology of University Hospital of Modena (Italy) between May 2006 and December 2010. All tests performed in both inpatients and outpatients were included in the analysis. Results: A total of 10,812 tests were performed in 8,623 individuals (mean age ± SD: 46±23 years; 52.8% male). Among subjects who underwent single testing (n=7,039, 81.6%), 69.7% were negative, 22.5% positive, 7.4% indeterminate and 0.1% not interpretable due to high background in the negative control (0.3% of all tests were not performed due to missing or inadequate samples). In 1,584 (18.4%) subjects (mean age ± SD: 41±23 years; 56.1% male) QFT-IT was repeated at least once: in this group, there was a significantly higher proportion of indeterminate QFT-IT results (11.6%) at first testing, as compared to subjects with single tests (p<0.0001). In most cases (72.5%) the second QFT-IT provided valid concordant results (53.5% concordant negative; 19.0% concordant positive); conversions (from negative to positive) and reversions (from positive to negative) occurred in 3.7% and 7.3% of patients, respectively. Indeterminate results were more likely to become negative instead of positive (7.1% vs. 0.8%; p<0.0001); 3.8% of negative results became indeterminate at second testing. Conclusions: In this non-experimental routine setting, repeated QFT-IT confirmed previous results in most cases, providing little additional clinical information. Indeterminate QFT-IT results were more frequently negative than positive at repeated testing, thus suggesting that most indeterminate tests do not mask a positive result.