My understanding is:
If PCR is done reliably it will pick up active virus replication during infection or reactivation in someone who is immunocompromised. But it may miss recent viral replication if there is no virus in blood.
Serological tests were done historically before PCR existed. The rule was that you interpreted two samples, usually two weeks apart. It was the change in the levels that was useful, much more than a single measure. If there was active infection you might see IgM rise or IgG rise a bit later.
For a time there was a suggestion that a single IgM level being raised was an indicator of recent or active infection, and more so for particular antigens (I think VCA). But it was then shown that single values do not reliably show recent infection. IgM levels may persist over time. (Worth remembering that between individuals viral antibody titres vary hugely so it is hard to define a useful normal range. It makes much more sense to use yourself as a control and look at changes over time.)
My reading of the situation is that for any one individual the presence of IgM and IgG antibodies to EBV VCA really only shows that there has been past infection. Some physicians (generally not virologists) have tried to use serology to diagnose active infection or reactivation and no doubt labs will be happy to sell testing on that basis. But I don't think it means much.
My understanding also is that when EBV antibodies have been looked at in ME/CFS populations and controls not much difference shows up if at all. There will be people with 'high' levels in any sample.
