01/19/2022
Does a positive COVID test mean that I'm contagious?
If you get a positive antigen rapid test, it's best to assume that you are contagious.
A positive PCR test doesn’t tell you whether you’re contagious, because PCR can be positive for weeks after the infectious period.
Neither PCR nor rapid tests directly measure contagiousness, but they both shed light on what’s going on inside you. The key is to understand what they can, and cannot, tell you.
RAPID TESTS
🧪 Rapid tests detect specific proteins inside the SARS-CoV-2 virus, and only light up when there is a ton of it on your swab, to the tune of millions of copies. Thus, a positive rapid test means that your swab is teeming with virus, and you are likely contagious.
⚠️ A negative rapid test is trickier to interpret. In the early days of an Omicron infection, you can be contagious yet test negative, because the virus hasn't taken off *yet* in your nose. In the later days of infection, a change from positive to negative sends a strong message that you've cleared the virus.
⏰ How long will I be positive? With rapid antigen tests, people only test positive during the infectious period of 1-2 weeks. A recent study of NBA players tested daily found high viral levels (likely contagious) for an average of 10 days with Omicron, and 11 days for the Delta variant, with significant person-to-person variation.
PCR
🧪 PCR can detect whispers of residual virus, long after the contagious period. Think of PCR as a sleuth checking out the scene of the crime to see if there is any lingering evidence of a villain. Thus, a positive PCR test doesn’t tell you whether you’re teeming with virus, or cleaning up the last little remnants. Your contagiousness depends on where you are in this journey.
⚠️ A negative PCR test means that your swab didn’t have even a small amount of virus. Just like a negative rapid test, this is not necessarily a green light, especially in the first few days of an infection, when the virus may not *yet* have taken off in your nose. False negatives PCR tests can also happen due to issues with sampling, processing, or analysis.
⏰ How long will I be positive? With PCR, you can expect to test positive for weeks, or even months (rarely), well after the infectious period.
🤓 Nerd Notes: PCR is super sensitive because it involves a signal “amplification” step (while rapid antigen tests do not). Viral RNA is first converted to DNA, then the DNA is copied many times in a process called polymerase chain reaction (PCR).
In reality, COVID PCR tests don't just give “yes” or “no” answers. They give a value, called a Ct or cycle threshold value, that indicates many amplification cycles it took to get a strong signal. A PCR test is reported as positive when the Ct value is below a certain cutoff (e.g, Ct
