https://theconversation.com/coronavirus-tests-are-pretty-accurate-but-far-from-perfect-136671
"Collecting good samples is not easy" part.
I had somewhere numbers, can't find this paper, but in short, if you do anything else, "easier" than nasal swab - it is much worse results (higher chance of false negative).
There is one more precise method, bronchoalveolar lavage, but you will not like it at all.