Researchers at Johns Hopkins found that the chance of a false negative result — when a virus is not detected in a person who is infected with COVID-19 — is greater than 1 in 5 and, at times, far higher.
Timing of the test determines the accuracy, and studies show that you should use caution when interpreting negative test results, particularly if you have been exposed or have symptoms consistent with COVID-19.
Source: ScienceDaily May 26, 2020