Comment by OneDeuxTriSeiGo

It's not really the same thing. If we exclusively tested coronavirus with PCR tests it'd be a lot more similar but the presence of kit tests and "30 minute covid testing" really muddied the water.

And of course Covid tests are primarily mucous membrane based which is going to be inherently harder to evenly test compared to a blood sample where viral load is pretty evenly distributed.

At least for blood based diseases, detecting viral load via PCR testing is to such a sensitivity that if there's essentially any active viruses out and about or any active viral RNA floating around in cells then the tests come back positive.

And with sustained antiviral use testing is less about "do I have it/will I become contagious in the near future" (like coronavirus) and rather "is my antiviral regimen still killing the virus faster than it can wake up from latent genetic material sleeping in DNA".

The former is a timing problem from one shot testing the latter is monitoring a steady state to track that the treatment remains effective and when it ceases being effective there's a lag time between viral load being detectable and sufficient viral load to be meaningfully contagious.