A positive test is a more accurate index than a negative test.  

Tests often miss the markers they're intended to find . However, the presence of what ever in a single test is not dipositive either. A series of tests, the number required varies with infection, are required to determine infection or lack.

I would think that anybody who tested positive for a cummunicable disease would isolate until a determination was made with a second, third test indicated infection or none.

Trump is a completely self-interested, sociopath type personality. Listen to him talk- he is wonderful, all the failures are somebody else.