Why don’t you need a negative coronavirus test to leave isolation?

President Donald Trump’s doctor on Saturday said he’s met criteria from the US Centers for Disease Control and Prevention to leave isolation. Those criteria don’t generally require a negative test for coronavirus. 

Here’s why:

People can continue to test positive even though they’re no longer infectious

Research has shown that PCR tests can still pick up pieces of genetic material from the virus long after someone has recovered. But these people aren’t likely to be infectious 10 to 20 days after symptoms began, according to the CDC.

To figure that out, scientists have taken samples from coronavirus patients and tried to infect living cells. Even though PCR tests can come back positive, patients don’t tend to be infectious after that 10 to 20 day window has passed, research has found.

Think of it this way: A PCR test is looking for the blueprint of the virus — its “genome” — and not the virus itself. In fact, the test is just looking for fragments of that blueprint. It’s like a recipe for chocolate cake; finding the recipe in someone’s kitchen doesn’t mean you’ll find a cake.

Why might Trump not need to isolate for 20 days?

For patients with severe Covid-19, the CDC says up to 20 days of isolation “may be warranted.” But the agency’s recommendations only require that at least 10 days have passed since symptoms first appeared.

“Consider consultation with infection control experts,” the CDC’s recommendations say. The President’s physician, Dr. Sean Conley, released a memo Saturday that referenced “advanced diagnostic tests” and stated “there is no longer evidence of actively replicating virus” from Trump.

Still, the letter didn’t fully describe those advanced diagnostic tests or their exact findings. Conley also didn’t disclose other vital signs from the President, such as his current oxygen levels — leaving many questions about Trump’s current condition unanswered.

Read more about Trump’s coronavirus status here: