4/14/2024 0 Comments Normal negative tb test pictures![]() ![]() The general principles for setting the frequency of surveillance TB testing in these situations are outlined below, however you are advised to contact APHA to discuss your individual circumstances. There are situations where a keeper has cattle herds located in more than one bTB risk area. ![]() TB testing regimes across different bTB risk areas in England The equivalent values for test reactors in the Edge Area and Low Risk Area (LRA) are marginally lower, but still quite high (approximately 88% and 77%, respectively). bovis, even when such animals show no visible lesions of TB at slaughter. This means that the vast majority of skin test reactors that are removed in that part of the country represent true infection with M. It has been estimated that the positive predictive value of the skin test read at standard interpretation is approximately 92% in the HRA of England. we are confident that the test is a strong indicator of infection. In this situation we would say that ‘the test has a high positive predictive value’ i.e. bovis as there is a high level of disease present in the area and we know that the probability of false positive results is very low. If we skin test an Officially TB Free (OTF) herd located in the High Risk Area (HRA) and a reactor is disclosed, it is highly likely that the herd is truly infected with M. Thinking about positive predictive value in the context of screening cattle for TB, we are essentially asking ‘If a skin test discloses reactors, what is the probability that they are truly infected with M. Sensitivity and specificity are purely characteristics of the diagnostic test itself, whereas positive and negative predictive values are a function of the sensitivity and specificity of the test and are also influenced by the level of disease in the population being tested. Positive and negative predictive values can be confused with sensitivity and specificity, but they are not the same. ![]()
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