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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%...

There are two general categories of tests typically used for identifying viral infections: those based on viral genetic material and those based on host immune response. The CDC page on MERS-CoV testing (also a coronavirus, just not the one that causes COVID-19) is an informative resource to start with (bold added and reformatted by me): Molecular tests are ...

From Time: June 3, 2020 Wuhan Tests Nearly 10 Million People in 19 Days, Finding Just 300 Coronavirus Infections (BEIJING) — The Chinese city of Wuhan, where the coronavirus was first detected late last year, has tested nearly 10 million people in an unprecedented 19-day campaign to check an entire city. It identified just 300 positive cases, ...

The results are probably in the Chinese press, but are being reported elsewhere China has reported 51 new coronavirus cases including 40 asymptomatic infections, majority of them in the contagion’s first epicentre Wuhan, where over six million tests have been conducted in the last 10 days, health officials said on Monday. So, looks like large scale ...

I cannot answer about South Korea, but as far as other drive-in test stations are concerned the rules I've seen are: Call ahead to get checked whether testing is indicated, get issued a special testing number. Go there, get tested with the testing number. Go back home, get test result via mail.

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-...

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. &...

The rt-PCR tests for Covid-19 are not 100% specific so yes it's possible to get a false positive. From the China PCR test The sensitivity of the RT-PCR diagnostic test was estimated to be 0.777 (95% CI: 0.715, 0.849), while the specificity was 0.988 (95% CI: 0.933, 1.000). The confidence intervals include sampling error in addition to the error due to ...

Well, this is the explanation I found in a (different) CDC paper: First, we based assay designs on previous diagnostic assays that had been developed for detection of MERS-CoV (12) and SARS-CoV (10) and targeted the N gene. Because of the relative abundance of N gene subgenomic mRNA produced during virus replication (13), rRT-PCR assays targeting the N ...

Well, interpreting the Charité numbers is still a bit of a mystery to me, but by analogy with a US case, having some tests close to the detection limit for Covid-19 is not so unusual in some circumstances. For the first (well-investigated) US patient, the Ct was below 30 in some tests (but around 20 in others), albeit this is with the CDC tests of the time: ...