How the PCR test works and why it’s such a critical weapon against Covid-19

The PCR test is akin to a companion diagnostic test: it makes much sense to do it alone in presence of something suspicious (like symptoms). If you're asymptomatic, it can be close to useless, because it does not mean there's infectious virus in your throat[1]. That is because you could be either presymptomatic (in this case, there would be infectious virus), or have cleared the infection already (in this case, not). In the latter case, presence of RNA does not equate to live virus automatically.

For asymptomatic people, antigen tests (especially the rapid kind, assuming they're sensitive enough) are a much better deal, despite their lower sensitivity, because they detect virus proteins, which are much likely indication of an infectious virus.

[1] https://www.cebm.net/covid-19/infectious-positive-pcr-test-r...