The creator of the PCR method (not really a test, but a method) was Kary Mullis.  He said that PCR is not capable of diagnosis. IOW: It is a methodology that is quantitative only.  There is no qualitative aspect to it.  Material is amplified by a pre-determined (and arbitrary) number of cycles until it is "positive" in the sense that there is enough material to study and analyze.  Change the number of cycles and you change the amount of material. This has nothing whatsoever to do with a "positive" diagnosis that someone has or is sick because of X. Mullis was quite vocal about this fact.  He cited the fraudulent application of PCR to HIV testing, where someone would claim a positive/negative diagnosis.  He said such conclusions are meaningless when PCR is the method. He died in August 2019.