Needing a PCR within 90 days of a positive PCR result

[Getyourheadaroundthis]

The guidance states you should not take a LFD or PCR within 90 days of testing positive for Covid unless you develop new symptoms. Reason being PCR can detect Covid even though you’re no longer infectious.

What remains unclear is - if you develop new symptoms let’s just say them being a high temperature and cough how does the PCR know whether they are due to new infection or are indeed late onset from the first infection? How accurate are follow up PCR tests? Do they have a threshold that indicate whether it’s old or new or infection or not? Has anyone experienced this? Anyone know how it works?

I think if you have symptoms and a positive test you will be told to self-isolate even though it won't be definite that you have a new infection. I recently had a PCR having had Covid last month and it was negative but my DD was still positive. DD hasn't had the vaccine and was more ill than me - not sure if that's the reason for her continuing to be positive.