[Ola]

From the web-

False negatives can occur for a number of reasons, including people being tested too soon after exposure to the virus (which may not let enough of the virus build up to a level that is detectable).

Our study showed that some patients with acute COVID-19 may test repeatedly negative by nasopharyngeal swab PCR. These cases should be interpreted as a low viral load in the upper respiratory tract rather than false negativity of PCR.