This story got me interest in testing so I took a little spin over to the CDC's COVID testing FAQ. It seems that testing is the wild, wild west. There are hundreds of labs offering tests, the VAST majority haven't even by FDA Authorized and none are FDA approved. 174 have been "removed" from the market - but who knows if they were actually recalled.
The specific test used by Musk, BD Veritor, specifically says it must be administered within the first 5 days of symptoms. It also warns about false negatives. It specifically says:
Antigen tests are very specific for the virus, but are not as sensitive as molecular tests. This means that a positive result is highly accurate, but a negative result does not rule out infection.
The instructions for collecting the sample also only says to swab 1 inch inside the nostril. More comfortable for the patient but less likely to collect the virus than the tests that swab WAY back in the nasal passage.
CDC FAQ
https://www.fda.gov/medical-devices/...#testingsupply
Musk's Test:
Medial Provider Instructions:
https://www.fda.gov/media/139753/download
Patient Instructions:
https://www.fda.gov/media/139754/download