PSA: A negative test result does NOT mean you're COVID-free!

It means that you don't YET have enough virus in your body to detect.

If you were infected right BEFORE the test, the virus might be there, but growing slowly
Or you might've caught the virus AFTER you took the test.

The CDC says "the incubation period for the virus can be up to 14 days and unless you were tested on the 14th day from your exposure, a negative test earlier in the quarantine period does not mean you are not infected."

http://publichealth.lacounty.gov/media/coronavirus/docs/about/GuidanceTestResults.pdf

MIT cites Kucircka et al:

"during the four days of infection prior to symptom onset, the probability of a false negative on the PCR test went from 100 percent on Day 1 to 67 percent on Day 4 [...] dropping to 20 percent three days after symptom onset."

https://medical.mit.edu/covid-19-updates/2020/07/when-should-i-be-tested

If you got infected even 5 or more *days* before your test, you may not have enough virus to show up on the test if your incubation period is longer than the median, which is pretty likely!

And even then you could be infectious and part of the 20% of false negatives.