evidence: PCR tests work by detecting a virus’ genetic material in biological samples like blood or saliva. These tests are highly sensitive and specific. Trace amounts of viral genetic material, left behind after an initial infection is over, can still be detected by PCR tests even when the infected person is no longer contagious. While PCR tests are excellent at detecting current and recent infections, there can be uncertainty when it comes to using them to determine if a person is still contagious.