Constance from Astoria, NY asks: “I tested positive for COVID-19, took Paxlovid, and isolated for 12 days. My symptoms are gone. Why does my PCR test show I’m positive?” Great question, Constance!
PCR tests are very sensitive, picking up even old fragments of viral RNA that may be leftover in your body but not capable of making more virus copies or causing infection. Many people stay PCR positive for several weeks (or rarely months) after an infection.
Rapid antigen tests work differently and test for a protein that is more likely to reflect active viral replication- thus the consensus that rapid antigens are a better (though not perfect) indicator of current infectiousness than PCR tests.
PCR tests may catch a new infection a couple of days before an antigen test (since it amplifies even small amounts of viral RNA), but the downside is they can continue showing positive even after there is no longer any viable virus.
We’re so glad you are feeling better Constance! It’s quite likely that your PCR test is picking up old bits of virus that your immune system has already smashed up. The vast majority (but not all) of people are negative on an antigen test 10 days after symptom onset.
So in general a negative rapid antigen test towards the end of your isolation can help raise confidence that you are no longer contagious, but a PCR test is not as useful for that purpose.
Stay safe, Stay well-
Those Nerdy Girls