Assessment of defined antigens for the diagnosis of bovine tuberculosis in skin test‐reactor cattle

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Abstract
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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