If I’ve been exposed, how soon should I be tested for COVID-19?
The short answer is: after known exposure, it is technically not very helpful to test for COVID until you develop symptoms. With the incubation period extending up to 14 days, prior to that 14-day point a negative test doesn’t tell us much as it could still turn positive later. If you develop symptoms, a test can then help to determine if you should continue on the 14-day quarantine path or if you should switch to the isolation and care recommendations for those who have been infected.
When is your test, on average, most likely to be positive if you are infected? The answer to this is sometime between 5 and 8 days after exposure. We know that 50% of people who will develop symptoms do so by day 5 and we know that test sensitivity for detecting infection begins to decrease again sometime after day 8.
While most people who develop symptoms and a positive test do so within this time period, not all do. You won’t know for sure that you’re not infectious until 14 days after exposure. Even if your test at the 7-day mark, for example, was negative.
This is why the CDC recommends a 14-day quarantine after known COVID exposure. Basing your contagiousness on symptoms or on a negative test done too early can cause you to unwittingly transmit infection to others. We know that many people begin to shed virus up to a few days prior to developing symptoms and we also know that a good percentage of people remain asymptomatic even though they have been infected. We know some asymptomatic infected patients do transmit the virus to others without knowing it. We know that those who eventually develop symptoms can transmit the virus up to a few days before they even know they’re sick. And we know that those with a negative test done earlier than 14 days after exposure can still develop infection and transmit it to others, up to 14 days after exposure.
So, to be safest to those around you, the best thing to do after a known COVID exposure is to hunker down and settle in for some quality quarantine time.
Jennifer Abrams, MD, October 5, 2020
Stanford University (undergrad)
Stanford University (med school)
Stanford University (residency)