That's not the only possibility. The number could also be predefined to a level making the tests too sensitive (which incidentally is the case in many countries

That's not the only possibility. The number could also be predefined to a level making the tests too sensitive (which incidentally is the case in many countries, currently). If you have absolutely no virus particles the test couldn't detect them no matter how many cycles you go through. Generally in the past, it was accepted practice that doctors do specific tests to get more certainty with diagnosing certain infections in a patient that's showing symptoms matching that disease. You could make a case for mass testing healthy people for all kinds of infectious diseases but for economical reasons I doubt many would support it. If we did that we'd have to somehow account for all the false positives, then.