It seems like Abbott's ID Now test works by detecting the virus directly as opposed to checking for antibodies? If the test result is negative does that mean you don't have the virus? Or does that not rule out a recent infection with the virus that has not yet reached a level high enough to be detected?
This is a great biological question! It asks a lot about how empirical science is done in the field of modern biology! I'm glad we encourage such questions from curious people who want to learn more.
It seems like Abbott's ID Now test works by detecting the virus directly as opposed to checking for antibodies?
The LOD described in the product insert is the lowest amount of viral genome in the collected sample that you need to get a true positive in 95% or more of administered tests, in units of "genome equivalents per mL".
This is effectively the minimum number of copies of viral genome per mL of assayed sample needed to give a true positive in 95% or more tests. Its LOD (125 copies/mL) is comparable with other tests (example).
A false negative can result if there is insufficient virus copies in the sample, or if there is test material contamination or the test is not performed correctly.
This assay includes positive and negative controls, which help to identify potential problems with how the assay was performed. Controls are important, given the many millions of tests that will be done to identify infected carriers of SARS-CoV-2 — the assay must be done correctly and without contaminants, in order to give the best statistical odds for the correct diagnosis of those who will be tested.