processed in a lab. They are capable of detecting tiny bits and pieces of the virus’s genetic material by copying whatever is floating around in a person’s sample over and over, amplifying it “a million- to a trillion-fold,” says Dr. Marie Louise Landry, director of the Clinical Virology Laboratory at Yale New Haven Hospital. That’s why, she explains, “PCR can detect very low levels of viral RNA for weeks and even months after infection, when a patient is no longer infectious.”
Rapid antigen tests, on the other hand, look for viral proteins rather than the virus’s genetic material. Many of them look for nucleocapsid or “N” proteins, which are abundant in infected cells and form a protective capsule around the virus’s genetic material. Unlike molecular tests, rapid antigen tests work only with what’s available — no copying or amplifying involved. These tests drag a person’s sample across a special piece of paper that contains a fence of antibodies designed to grab onto the virus’s N proteins. If enough proteins snag on the fence, a visible line of color will appear.
“Even single molecules can turn a PCR positive,” says Dr. Michael Mina, an epidemiologist who has been a vocal supporter of rapid testing throughout the pandemic and is now chief science officer for the health care company eMed. But, he added, it takes “about 100,000-1,000,000 molecules to turn the rapid antigen test positive.”
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Should you continue isolating if you test positive?
But what about the official recommendation that people with COVID should isolate for five days at home — followed by five days of masking? Data from the world of sports, where resources and testing methods abound, offer a glimpse into how variable that period of infectiousness might be.
A preliminary study of NBA players and employees found that among 70 people infected with Omicron, by Day 5 after their first positive test about 40% of them were still likely infectious.
“What we’re recommending is that between Days 5 and 10, you can take an antigen test. If it’s positive, you stay in isolation till Day 10,” Binnicker suggests.
CDC public affairs specialist Jasmine Reed says the best approach is to use an antigen test toward the end of the five-day isolation period if the person’s symptoms have improved. “If your test result is positive, you should continue to isolate until Day 10,” she adds.
Wroblewski says an important gauge of contagiousness is the simplest: how someone is feeling.
“If you have a high fever and a cough, don’t go see people. I feel like we somehow forgot that part,” she shares. “Let’s not put so much emphasis on the test and the technology that we forget basic infection control practices: that if you’re sick, stay home.”