The performance of the interferon gamma assay when used as a diagnostic or quality assurance test in Mycobacterium bovis infected herds

There are two different contexts in the Irish bTB eradication programme in which the interferon-gamma assay (IFN-gamma) is applied. Firstly, the IFN-gamma assay is applied routinely to high risk cohorts in herds with four or more reactors to the SICTT. The IFN-gamma test is then carried out on blood samples submitted to the laboratory within 8 h of collection (diagnostic testing). Secondly, the use of the IFN-gamma assay has recently been extended to test SICTT reactors as part of a general quality assurance (QA) scheme to monitor the performance of the SICTT. Blood samples from reactors are tested one day after blood collection (QA testing). In this study, we analysed the relative performance of the SICTT and IFN-gamma when used in parallel as an 8 h diagnostic test and as a 24 h QA test on SICTT reactors. A total of 17,725 IFN-gamma tests were included in the analysis (11,658 diagnostic tests and 6067 QA tests). Of the samples submitted for diagnostic testing, the proportion positive to IFN-gamma decreased with the severity of interpretation of the SICTT result. Of the standard reactors that were tested with IFN-gamma in the QA programme, 92.2% were positive to the IFN-gamma test. Among animals that were SICTT -ve/IFN-gamma +ve, 18.9% were positive at post-mortem compared to 11.8% of those that were SICTT +ve(standard reactor)/IFN-gamma-ve. These results highlight the risk associated with retaining SICTT -ve/IFN-gamma +ve animals, and suggest that prompt removal of these animals is necessary to reduce the potential for future transmission. (C) 2017 The Authors. Published by Elsevier B.V.