The continued use of purified protein derivative (PPD) tuberculin is considered to be the main factor which limits the specificity of diagnostic tests for bovine tuberculosis (TB). This study evaluated a whole blood interferon-gamma (IFN-y) assay and compared the diagnostic potential of PPD with two tuberculosis-specific antigens, ESAT-6 and MPB70. To provide estimates of sensitivity and specificity, responses were measured in 180 skin test-reacting cattle, of which 131 were confirmed as tuberculous, and in 128 cattle from TB-free herds. For the skin test reactors, there was a positive correlation between the IFN-y responses to PPD from Mycobacterium bovis (PPDB) and PPD from Mycobaderium avium (PPDA), indicating cross-reactivity between these conmplex antigens which are the basis of the skin test. In comparisons of the ESAT-6 IFN-y test with a PPD IFN-y test (using PPDB compared with PPDA), there was a decrease in sensitivity (76-3 per cent vs 89-3 per cent), but a clear increase in specificity (99-2 per cent vs 92-2 per cent).
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