Isn't half the selling point of antiretroviral therapy that you're no longer contagious?

https://i-base.info/u-equals-u/

U=U probably does not apply to all diseases for the reasons you mentioned though.

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.

It depends on why you're testing. You never just test to gather data without purpose. So when we were doing PCR testing, it was partly about gathering population statistics to build dashboards, because a surge in positive PCR test results would correlate to a subsequent surge in the hospitalization rate. The hospitalization rate and the percentage of empty versus full beds were what needed to be managed, as an increase in severe Covid patients would overwhelm hospitals making it impossible to treat people with otherwise routine emergencies like arterial wounds. There were a lot of videos out of India in late 2020 which showed the result of uncontrolled Covid surges, and it was mostly Hospital Full -> No treatment for anyone -> Lots of deaths. Or a lot of sick and injured people waiting outside or taking welding gas because the hospital grade oxygen was used up.

By contrast, testing with rapid tests is mostly about seeing why you're sick before you go somewhere, and those tests probably don't need to be as sensitive. The hospital has capacity to treat people because of the PCR tests and so we can manage if some of the PCR tests are false-negative. Most people are vaccinated so we can tolerate some Covid-positive people mingling in public places now.

Tests aren't magical, and there are different tests with different capabilities. I'd suggest you learn about sensitivity versus specificity and learn how to compute the false positive, false negative, true positive, and true negative rates. It might be eye opening for you.