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The ratio of false positives to true positives (as well as overall "accuracy") is a function of the underlying rate in the population. Let's imagine you have a test that has 95% specificity. Specificity is the number of negatives that you correctly identify as negative. If you test 1000 negative samples with a 95% specificity test, you will find 95%...


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Presymptomatic patients with coronavirus may test positive with RT PCR (see: Kim, Jeong et al. Viral kinetics of SARS-CoV-2 in asymptomatic carriers and presymptomatic patients. Int J Inf Dis Vol.95, P441-443, June 01, 2020.) Presymptomatic patients may also be infectious (see: Bai Y, Yao L, Wei T, et al. Presumed Asymptomatic Carrier Transmission of COVID-...


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Bryan is correct, especially in his comment in the case of 0% true/prior prevalence 40/1000 = 4% positive. 1-2/40 = 2.5-5%. But even in such a conditioned test scenario, in which the 2nd test is only applied to the positive-reporting sample of the first test, a decent test (e.g. 90% sensitivity, 95% specificity) will actually converge pretty quickly (i.e. &...


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