Jan 14, 2022 4:00 PM

Author: University of Utah Health Communications

There are two types of tests: PCR and antigen. They both test for the SARS-CoV-2 virus, but there are differences. Both of these tests can be useful, but it can be hard to know which is best for your particular circumstances. 

Emily Spivak, MD, MHS, an associate professor in the Division of Infectious Diseases at University of Utah Health, offers the following information about how to test, when to test, and which test is best. 

What is the difference between PCR and antigen test?

PCR test detects viral RNA. It’s the type of test you usually take at a testing center, where testers will either use a nasal swab or your saliva for determine if you are positive for COVID-19. The turnaround time will vary depending on testing capabilities in your area. But during a surge in cases, it can take anywhere from 24 hours to several days to get results. 

Antigen tests (rapid tests) detect proteins on the surface of virus. These tests are available at testing centers and in over-the-counter kits. Results for these tests may come back in as little as 15 minutes.

If you suspect you have COVID-19, should you get a PCR test or an antigen (rapid) test?

Ideally a PCR because it’s more sensitive and accurate, and more likely to detect the COVID-19 virus. Depending on comunity spread in your area, it can take days to get a result. If this is the case in your area, a rapid home testing kit is useful given good performance during the symptomatic phase of illness, especially after 12 to 24 hours of symptoms.

If you’ve been exposed to COVID-19, on what day should you get tested?

The recommendation is three to five days after exposure. If you’re symptomatic, sooner. If you’ve had a high-risk exposure and a rapid test comes back negative, it’s best to follow up with a PCR test to truly rule out COVID-19 infection. 

Can you skip testing and simply assume you have the virus if you’re experiencing symptoms?

It’s always best to test, especially if you're at high risk of severe disease, work in health care, or interact with vulnerable populations. But if COVID-19 rates are high in your area, it’s reasonable to assume you have it if you have symptoms. If you’re sick—even if you don’t have COVID-19—staying home is a good idea. Whether it’s COVID-19, the flu, or other infectious diseases, please isolate if possible and always wear a quality mask if you must go out in public.  

Are over-the-counter rapid tests as accurate in detecting COVID-19 as other methods?

Rapid tests are less sensitive than PCR tests, but they have a sensitivity rate of roughly 90 percent when people are symptomatic. They are less sensitive in asymptomatic individuals. And if you use a rapid test too early, less virus may be present, therefore, leading to a negative result. If you test early with symptoms and the result is negative but you suspect you have COVID-19, repeat the test 24 to 48 hours later or pursue PCR testing. 

Some people are sharing on social media that you should swab your throat instead of nose if using a home testing kit. Is that recommended?

Some unpublished data suggest that, early in infection, a sore throat is a common symptom of COVID-19. And viral replication with Omicron may start at high levels in the throat and move up to the nasal passages later. This may make nasal tests negative early in symptoms. At this time, I'd recommend people try and pursue saliva PCR testing early in symptom onset or use a repeated rapid antigen testing strategy. 

People are also sharing, anecdotally, that you may get a better result by swabbing the throat or swabbing the throat first, then swabbing the nostrils. This is not currently recommended by test manufacturers. Information is still emerging, and it needs to be studied before it can be recommended.

Are rapid tests less accurate in detecting the Omicron virus?

It's not known yet. Studies are still forthcoming, but from is known thus far are that rapid tests are still good at detecting Omicron. However, they may be less accurate on the first day of symptoms. You should assume you have COVID-19 and wait to test on day two of symptoms or repeat testing if initially negative. 

How accurate are PCR tests? Some people who test negative on a PCR test are still convinced they have COVID-19.

PCR tests are highly accurate, but they’re not 100 percent accurate. They’re also subject to the accuracy of the sample that is collected. If you get a negative result but have symptoms of COVID-19 illness, isolate at home and try to repeat the test in 24 hours. Also realize that other respiratory viruses are circulating in the community, and it's important to stay home to stop transmission of other viruses. 

If you had COVID-19 and are now symptom-free after five days , do you need to test again? Or can you safely return to work with a quality mask?

Talk to your employer about the requirements in your workplace. Current CDC guidance recommends that if you can test, do so, but it’s not required. If you are symptom-free, you can return to work, but should wear a quality mask, such as an KN95, for at least five more days. If after five days you still have a fever or significant cough, don’t come out of isolation until your symptoms clear up. It’s important to note that a cough may linger for days or weeks, so use discretion if your cough is productive and frequent. But if it’s a dry, intermittent cough and your other symptoms have cleared, it’s likely safe to come out of isolation—as long as you can continue to wear a mask. 

My main advice is to vaccinate, get your booster, use a high-quality mask such as a KN95, practice social distancing, work from home if you can, and test if you suspect you have the virus. If you can’t test but feel sick, isolate if you can.

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