Reposting, LB too late.

How does a PCR Test for Coronavirus work:

RNA isolated and purified from upper and lower respiratory specimens is reverse transcribed to cDNA

and subsequently amplified in the Applied Biosystems 7500 Fast Dx Real-Time PCR Instrument with SDS

version 1.4 software. In the process, the probe anneals to a specific target sequence located between

the forward and reverse primers. During the extension phase of the PCR cycle, the 5’ nuclease activity of

Taq polymerase degrades the probe, causing the reporter dye to separate from the quencher dye,

generating a fluorescent signal. With each cycle, additional reporter dye molecules are cleaved from

their respective probes, increasing the fluorescence intensity. Fluorescence intensity is monitored at

each PCR cycle by Applied Biosystems 7500 Fast Dx Real-Time PCR System with SDS version 1.4 software.

Sauce:

https://web.archive.org/web/20200630142614/https://www.fda.gov/media/134922/download

Advantages and Limitations of PCR Testing:

Although PCR is a valuable technique, it does have limitations. Because PCR is a highly sensitive technique, any form of contamination of the sample by even trace amounts of DNA can produce misleading results (Bolognia et al, 2008; Smith & Osborn, 2009). In addition, in order to design primers for PCR, some prior sequence data is needed. Therefore, PCR can only be used to identify the presence or absence of a known pathogen or gene. Another limitation is that the primers used for PCR can anneal non-specifically to sequences that are similar, but not completely identical to target DNA. In addition, incorrect nucleotides can be incorporated into the PCR sequence by the DNA polymerase, albeit at a very low rate.

Sauce:

https://web.archive.org/web/20200630134027/https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4102308/

>>9799328

Yes. It also may falsely recognize pathogens "similar" to the one being tested for.

>>9799368

"Another limitation is that the primers used for PCR can anneal non-specifically to sequences that are similar, but not completely identical to target DNA."

>>9799243

Yes. A former colleague of Mullis (Inventor of PCR) stated in an interview that Mullis has been pretty clear about his view that PCR is a manufacturing tool and not really suitable for diagnostics.

This is probably because, as you said, it'll amplify even tiniest trace amounts of DNA or, in case of RT-PCR, RNA present.

>>9799529

Here's the exact quote from the interview:

"PCR is really a manufacturing technique,” Crowe explained. “You start with one molecule. You start with a small amount of DNA and on each cycle the amount doubles, which doesn’t sound like that much, but if you, if you double 30 times, you get approximately a billion times more material than you started with. So as a manufacturing technique, it’s great. What they do is they attach a fluorescent molecule to the RNA as they produce it. You shine a light at one wavelength, and you get a response, you get light sent back at a different wavelength. So, they measure the amount of light that comes back and that’s their surrogate for how much DNA there is. I’m using the word DNA. There’s a step in RT- PCR test which is where you convert the RNA to DNA. So, the PCR test is actually not using the viral RNA. It’s using DNA, but it’s like the complimentary RNA. So logically it’s the same thing, but it can be confusing. Like why am I suddenly talking about DNA? Basically, there’s a certain number of cycles.”

Source:

https://web.archive.org/web/20200630150102/https://www.leanmachine.net.au/healthblog/was-the-covid-19-test-meant-to-detect-a-virus/

>>9799608

To be fair, there is currently no other/better way to detect the presence of the viral RNA.

Antibody tests, on the other hand, do not check for the presence of the pathogen, but for the presence of an immune system response to the pathogen, i.e. if you had the virus. But the accuracy of this method is questionable as well, from what I can remember.