See full answer The accuracy of any diagnostic test depends on many factors, including whether the sample was collected properly. For PCR tests, which are typically analyzed in a laboratory, test results may be affected by the conditions in which the test was shipped to the laboratory. Results may also be affected by the timing of the test. For example, if you are tested on the day you were infected, your test result is almost guaranteed to come back negative, because there are not yet enough viral particles in your nose or saliva to detect. The chance of getting a false negative test result decreases if you are tested a few days after you were infected, or a few days after you develop symptoms. Generally speaking, if a test result comes back positive, it is almost certain that the person is infected.