The interferon-gamma test is a supplementary blood test that has been used alongside the primary tuberculin skin test in Great Britain since 2006 in order to maximise the detection of TB-infected animals in certain herds affected by TB breakdowns.
No diagnostic test is perfectly accurate and there is a trade-off between the sensitivity and specificity of a test. The interferon-gamma test is more sensitive than the skin test, which means it is less likely to miss TB-infected animals (approximately 1 in 10 infected animals compared with 1 in 4 for the skin test). However, it is less specific than the skin test (on average 3-4 false positives per 100 disease-free animals tested). Even so, it is incorrect to assume that all interferon-gamma positive animals with no visible lesions of TB at slaughter represent false positive results.
That is why the interferon-gamma test is, with few exceptions, only used in fully confirmed (lesion and/or culture positive) TB breakdown herds. In such herds the predictive value of a positive interferon-gamma test result is maximised and the cost of taking out low numbers of false positive animals is outweighed by the benefit of earlier and more complete detection of all TB-infected animals in the herd. This in turn reduces the risks of repeat (recurrent) TB breakdowns in the same herd and spreading the disease to other herds as a result of the incomplete elimination of all the infected animals.
In 2018/2019, over £4 million has been allocated to Defra’s bovine TB research and development programme, which includes research on improved and novel diagnostic tests for bovine TB.