Throughout the pandemic, COVID-19 tests have served as essential tools for measuring the spread of the virus. Along with masking and vaccination, regular testing can prevent individuals with COVID-19 from unknowingly transmitting the virus to others.
The U.S. Food and Drug Administration (FDA) has authorized several COVID-19 diagnostic tests for emergency use during the pandemic. Two types of tests commonly used to detect COVID-19 infection are rapid antigen and PCR.
To explain the differences between these tests and offer recommendations for their use, Jodie Guest, PhD, professor and vice chair of the department of epidemiology at Rollins School of Public Health, teamed up with Colleen Kraft, MD, professor in the department of pathology and the department of medicine division of infectious diseases at Emory University School of Medicine. Kraft is also associate chief medical officer of Emory University Hospital, a part of Emory Healthcare.
Their conversation is part of an online video series hosted by Guest, who also leads the Emory COVID-19 Outbreak Response Team, answering questions related to the COVID-19 pandemic. Watch the full conversation between Guest and Kraft here.
Q: What are rapid antigen tests?
A: Rapid antigen tests are COVID-19 tests that can be self-administered at home and return results within 15 to 30 minutes. These tests work by detecting the presence of viral antigens on a nasal swab sample.
“For a list of these tests and their sensitivity information, you can go to the CDC website and look up antigen testing, and it will give you a very long list of what is approved under Emergency Use Authorization,” says Kraft. “Or you can actually go to the FDA website and see what they presented to the FDA and learn about what they are detecting.”
Q: When should rapid antigen tests be used for diagnosis?
A: “If you’ve had an exposure and then you become symptomatic, this is a really nice place to use these tests,” Guest says. It is important to follow CDC guidelines for quarantine between the time of the exposure and the test.
In asymptomatic individuals, rapid antigen tests are more likely to return a false negative result than PCR tests because they are less sensitive.
“If I had an exposure yesterday and I take an at-home antigen test today, that’s really not going to be able to detect whether or not I have contracted COVID-19,” Kraft explains, because a person’s viral load may not yet be high enough. “We want to make sure that people then don’t take that test and say, ‘Well, I’m negative, therefore I’m fine,’ and not continue to monitor for symptoms or receive a PCR test.”
“For exposure and no symptoms, sometimes it’s best to go somewhere and get a PCR test so you can be absolutely sure that you don’t have the virus,” she adds.
Q: How can people with COVID-19 use rapid antigen tests during isolation?
A: “Let’s assume that I am a person who has had a positive COVID-19 test,” Guest says. “The CDC guidance says that after a five-day isolation period, if I am not symptomatic at that point or have not been symptomatic the entire time, I can leave that isolation and then continue forward in public with five days of strict mask-wearing. The CDC does not require a test to leave isolation at that point in time.”
Guest notes that other countries do require a negative antigen test to trigger the end of an isolation period. “For example, the U.K. guidance uses an antigen test at day six, and then it needs to be followed by another negative antigen test at day seven — and they need to be separated by a full 24 hours for that to count.”
“I would be most comfortable making sure I come out of isolation having had a negative antigen test,” she continues. “I’m going to be particularly careful about it if I am in a position where I am around people who are going to be more vulnerable, to make sure that I’m not spreading it to them.”
Individuals who remain symptomatic at day five of isolation should continue to isolate until day 10, according to CDC guidelines. After that time, testing is not needed.
Q: What is a PCR test?
A: PCR (or polymerase chain reaction) tests can be up to 10 times more sensitive than rapid antigen tests. They detect genetic evidence of COVID-19 in a swab sample which is typically collected at a testing clinic.
“PCR takes that genetic material, and it looks specifically for it with something called primers. When it finds that, then it amplifies that up,” Kraft says. “That’s why we call them more sensitive, because it can actually detect that needle in a haystack.”
“You can be very, very sure that you don’t have COVID if you have a negative PCR test, because it is looking very deeply in that sample,” she adds.
Q: When should PCR tests be used for diagnosis?
A: Kraft recommends going to a clinic for PCR testing three to five days after an exposure to COVID-19. PCR tests can find the virus even at lower levels, “versus needing to be at the peak amount, which is where the antigen test works the best.”
While PCR tests are great for diagnosis, Kraft and Guest do not recommend people with COVID-19 use them to determine whether they are still contagious.
“Remembering that the ‘needle-in-a-haystack test’ is the PCR test, it’s going to find even dead virus that’s still being cleared by your body,” Kraft says. “So, we don’t recommend if you’ve had a positive PCR test to do anything on the other end to define clearance. At that point, we go a lot by symptoms and by the recent CDC guidance, especially that vaccinated individuals can clear this virus a lot sooner, which is why they also don’t have severe disease.”
“An antigen test is a great test after a PCR to sort of get you out of isolation and to determine whether or not you still have that amount of virus that can be contagious,” she adds.
Q: How can rapid antigen tests be used as tools for gathering safely with others?
A: Combined with COVID-19 mitigation measures, antigen tests can play an important role in preventing the spread of COVID-19 at indoor gatherings. Testing is especially important if any attendees are immunosuppressed or unvaccinated.
In preparation for a gathering such as a dinner party, individuals should mitigate potential exposure to COVID-19 as much as possible in the week leading up to the event. “Everybody needs to be masking, and not being in enclosed places without masks,” Kraft says. “So, you’re not going to be doing other things in advance of being at that party in case you have an asymptomatic case and could still spread it.”
All attendees should test as close to the time of the dinner party as possible. Guest also recommends testing multiple times in a sequential manner, if possible. This increases the sensitivity of the antigen tests.
“I like to use one test the day before I gather, and then one immediately before gathering. That is two tests in order to gather safely, and that is for everyone who is going to be part of the gathering,” Guest says. “The possibility that I would have a false negative two times in a row is statistically not very probable.”
Q: Can rapid antigen tests detect all COVID-19 variants?
A: With every new variant, there will be uncertainty about how effectively tests can detect the virus. Test manufacturers and even academic medical centers like Emory conduct studies comparing rapid antigen tests to PCR tests to see how well they pick up different strains of COVID, Kraft says.
“I have no concerns that these pick up Omicron very well,” she says of the tests currently on the market. “The other thing about Omicron is it’s creating a lot of virus, and that’s part of the reason it’s so transmissible. So, it’s fairly easy for antigen tests to work right now.”
Q: Can rapid antigen tests produce a false positive result?
A: “There is a chance of a false positive, but it’s almost not going to happen during this time when our community prevalence is about 20%,” Kraft says. “During this Omicron surge, we definitely would trust a positive result.”
As community prevalence drops, Kraft says the chances of a false positive result will increase. At that time, new infections are best diagnosed by PCR tests.
Q: Should I swab my throat with a rapid antigen test?
A: “There have been some headlines that suggest that instead of using your test swab in your nose — the way these have been studied and the way they are approved under Emergency Use Authorization — to actually swab your throat, instead,” Guest says. She cautions against this approach.
“Rapid antigen tests have not yet been well tested for that, so that is not how we want to use a test — where the data have not been well collected and examined by experts in the field,” she explains.
Kraft also recommends sticking with a nasal swab. “You don’t have to do the brain biopsy, but just do a really good swabbing of the inside of your nose,” she says. “That should be adequate — that is part of where COVID lives in our bodies.”
While the throat may be a good place to sample for COVID-19 infection, it can also be difficult to properly swab. Further study is required before experts can recommend this method for at-home antigen testing.
Q: How can I get free rapid antigen tests?
A: In the U.S., rapid antigen tests are now available to order online at COVIDtests.gov. Every household is eligible to receive four free tests delivered to their homes by the United States Postal Service.
“I filled out my application,” Guest says. “It’s very simple and straightforward, and I’m anticipating mine arriving soon. We really urge everyone to take advantage of being able to get some of these tests and have them in your home.”
Q: Can I go to the emergency room for a COVID-19 test?
A: The best place to take a COVID-19 test is at home or at a designated testing center — not at an emergency room.
“Please don’t use the emergency department to go get a COVID test,” Kraft says. “We are strapped. We are looking at a long haul of the quality of our health care going down because of people leaving health care — and being crushed all the time is very tiring.”
“You can help by either going to a community place that is doing antigen or PCR testing, or if you can afford it, please purchase your own antigen tests,” she continues. “Do not go to an emergency department unless you feel terrible and think that you may need hospitalization or some sort of emergency intervention.”