There is a difference between HIV vs SARS-Cov2: If you get AIDS, your body does not have the capacity to recover from it on its own. So, 1) the chances of detecting dead material from a long gone infection are nil; and 2) the cost of a false negative is much higher than the cost of a false positive.
Since most people seem not to be seriously affected by a SARS-Cov2 infection, neither is true for SARS-Cov2. So, not agreeing on a fixed, reasonable, standard number of cycles which everyone uses has the consequence of inflating false positives for no gain. Note WHO's updated information[1].
> WHO guidance Diagnostic testing for SARS-CoV-2 states that careful interpretation of weak positive results is needed (1). The cycle threshold (Ct) needed to detect virus is inversely proportional to the patient’s viral load.
If you are not sick and your viral load is barely detectable, what's the point?
See also Figure 1 in WHO's lab guidance[2].
[1]: https://www.who.int/news/item/20-01-2021-who-information-not...
[2]: https://apps.who.int/iris/bitstream/handle/10665/334254/WHO-...
Everyone should use the same number in all tests. From what I understand, about 20 is a reasonable number which might still result in positives up to a month post-infection. I consider having a standard more important that the specific number chosen. It seems when people go to > 30 it is in a quest to ensure a positive test result.
Also I’d only heard of people speak in terms of “positive” for pcr. You can be positive but not infectious, if there are a lot of cycles and you’re tested as the disease winds down.