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Bovine Tuberculosis: Disease Control

Volume 461: debated on Friday 15 June 2007

To ask the Secretary of State for Environment, Food and Rural Affairs what estimate he has made of the level of (a) false positives and (b) false negatives resulting from tuberculosis testing of the UK cattle population. (142136)

It is not possible to establish such data other than as part of a research programme where all animals are slaughtered and the disease status of each animal can be irrefutably established. No such technique is available but immunological tests are considered the best indicator of infected status.

There are two immunologically based diagnostic tests used in the Great Britain (GB) bovine tuberculosis (bTB) testing programme. The primary screening test for bTB in cattle in the UK (and Ireland) is the single intradermal comparative cervical tuberculin (SICCT) test, which is commonly known as the tuberculin “skin test”. In October 2006, the Government extended the use of the gamma interferon (g-IFN) diagnostic blood test, alongside the skin test (as permitted by EU legislation), in certain prescribed circumstances.

Research shows that when the skin test is applied to cattle in bTB-free herds in GB there is a one in 1,000 chance that a non-infected animal will be wrongly classified as a reactor. This is known as the test's false positive rate. An alternative way of defining this is to say that the skin test has a specificity of 99.9 per cent. Although the probability of getting at least one false positive result increases with the size of the herd being tested, it would be extremely rare to find more than one false positive in any herd. The skin test is designed to detect an immune response at a relatively early stage in the infection process. Therefore, the percentage of test reactors without visible tuberculous lesions is not an indicator of the false positive rate for this test.

Various studies have shown that the sensitivity of the skin test (i.e. its ability to identify infected animals as positives) varies between 77 per cent. and 95 per cent., i.e. it can be expected to miss about one or two in every 10 infected cattle on a single round of testing (10-20 per cent. false negative rate).

For all diagnostic tests there is a trade off between sensitivity and specificity, so different interpretations of the test can be used under different disease situations. Sensitivity is enhanced in herds with post-mortem or culture-confirmed infection by application of severe interpretation.

With regard to the g-IFN blood test, performance evaluation carried out in a number of countries shows that at the laboratory cut offs used in GB, it has a sensitivity comparable to or marginally better than that of the SICCT (between 73 and 100 per cent., with a median value of about 87 per cent.). Because the two tests detect slightly different sub-groups of infected cattle, by combining the two tests a higher overall sensitivity can be achieved. A trial in GB, established to evaluate the specificity of the blood test, confirmed the findings of previous studies by concluding that the commercially available blood test had a specificity of between 95-97 per cent. (i.e. there is a likelihood that slightly more false positive reactors are identified when the gamma test is applied, as opposed to the skin test). It is for these reasons that the g-IFN blood test cannot be used for routine surveillance and is best used in parallel in infected herds to maximise the detection of infected animals.