I'm not sure if this is it, but assuming the correctness of the test is independent of the sample quality, a sample could be split and tested multiple times to obtain a result with a much better FPR.
To my knowledge, the most common failure mode is that the concentration of viral RNA in the sample is below the limit of detection. This could be because the swab wasn't done correctly, the patient wasn't shedding very much virus, the sample was mishandled (heat can break down the sample), or the sample wasn't adequately mixed.
cameldrv|5 years ago
mehrdadn|5 years ago