Can someone test negative for the coronavirus but be a carrier?
Yes. Both nasopharyngeal swab test and blood test can return false negative.
Direct sample of the virus, from a throat or nose, may return false negative:
These tests are considered very accurate but in some cases if they miss the virus – say, because it is further down in the lungs and someone doesn’t cough up any phlegm (or sputum) – they can return a false negative.
Blood tests, which look for the body’s immune response to the infection, can also return false negative:
Antibodies deployed by the body to fight off the virus can take more than a week to form. They may not show up if the test is performed too early into the illness, or they could be mistaken for those used against another virus. This is also true of the rapid finger-prick tests - there has been fault tests and concerns about their overal sensitivity.