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 s

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.