What’s The Difference Between COVID-19 PCR Tests And Antigen Tests?

Along with vaccines, COVID-19 testing is a key tool to preventing the spread of COVID-19. It can also help you detect an illness in the early stages--which is especially important if you have an underlying health condition, as you might be eligible for outpatient COVID-19 treatments that can help prevent your illness from becoming severe. 

But there’s a lot of questions surrounding COVID-19 testing: Are tests accurate? What’s the difference between a PCR test and an antigen test? Which test should I take? When should I take a test? To clear up the confusion, Dennis Cunningham, M.D., medical director of infection prevention and control at Henry Ford Health, breaks down the facts about each test—and shares when to take each one.

COVID-19 PCR Tests

PCR tests are designed to detect COVID-19’s genetic material, so they can identify very small amounts of the virus in your body. PCR tests are administered by health professionals at hospitals, health clinics and drugstores. Tests are usually sent to a lab, so it can take a few days to process the results. Here’s what to know about PCR tests:

• PCR tests can detect COVID-19 variants. This includes both the Omicron and Delta variants.   
• They’re close to 100% accurate if you have symptoms. “PCR tests are the best tests out there,” says Dr. Cunningham. “If you have symptoms, they’re most accurate if you get tested within the first five days of symptoms.”
• If you don’t have symptoms, PCR tests lose a bit of accuracy. If you’ve had a COVID-19 exposure but don’t have symptoms, quarantine and test yourself within three to five days of exposure. If you’re symptom free, they may be around 88% accurate instead of 98% accurate, but they’re still by far the best tests available, says Dr. Cunningham.
• Because PCR tests can detect very small amounts of virus, if you take a PCR test after recovering from COVID-19, it could still give you a positive result. If it has been 10 days, you have fully recovered and you are symptom free, this doesn’t mean you’re still contagious. If you still have symptoms however, you could still be contagious.

COVID-19 Antigen Tests

Antigen tests are designed to detect certain proteins in COVID-19, including the main spike protein. Antigen tests are the free tests the government is giving out: Every home in the United States can order four, free at-home tests at covidtests.gov. Private health insurance companies will also cover the cost of at-home tests. (Check with your healthcare plan for details.)

Antigen tests are sometimes called rapid tests—and they are rapid, as they give you quick results—but it’s better to call them antigen tests because some PCR tests can also be rapid. Here’s everything you need to know before taking an antigen test:

• Antigen tests can detect COVID-19 variants. This includes both the Omicron and Delta variants.   
• For proper use of antigen tests, read the directions. They require you to swab each nostril in your nose. They give you tests results within a few minutes by revealing either one or two lines: one line means it’s negative, two lines means it’s positive.
• Antigen tests are most accurate if you have symptoms. “Antigen tests were designed to detect the virus in people with symptoms,” says Dr. Cunningham. “Depending upon the brand, antigen tests can be up to 96% sensitive, meaning if you have COVID-19 they will detect it. It’s best to test yourself within the first 48 hours of experiencing symptoms—the test will be most accurate during that time because your body hasn’t shed much of the virus yet."
• If you don’t have symptoms, the accuracy of antigen tests can drop to around 60%. If you’ve been exposed to COVID-19 and you don’t have symptoms, quarantine for five days and test yourself within three to five days of exposure.
• If an antigen test is positive, believe it. “With an antigen test, you are far more likely to receive a false negative than a false positive,” says Dr. Cunningham. “If it’s positive, you don’t need to get tested again at your doctor’s office or the hospital.” After receiving a positive test result, isolate until you are fully recovered. 
• If an antigen test is negative, don’t assume you’re negative. If you’ve had a COVID-19 exposure and receive a negative test result, it’s still important to quarantine for five days after testing negative. If you receive a negative test result and have symptoms, assume you have COVID-19. Quarantine and call your doctor or healthcare provider, who may decide to give you a PCR test.
• Store testing kits at room temperature, out of direct sunlight and away from a direct heat source. Testing kits come with an expiration date, but Dr. Cunningham says they’ll still be usable for a few months afterward.

When To Take A PCR Test Vs. Antigen Test

PCR tests are the gold standard, but antigen tests can be helpful, too, as long as you don’t throw caution to the wind. If you’ve had a COVID-19 exposure, you aren’t feeling sick and you have the ability to quarantine, take an antigen test—even if you receive a negative result you can stay away from others for a while.

If you are very sick and think you have COVID-19, take a PCR test. If you’ve had a COVID-19 exposure and you don’t feel that sick—but you aren’t able to quarantine for an extended period of time—taking a PCR test may also be best.  

At the end of the day, while testing is important, it’s not a cure-all. Because no test is 100% accurate, getting a negative test isn’t a green light to not wear a mask or social distance. Being up to date on your vaccinations (that means getting a COVID-19 booster shot), wearing masks and social distancing are essential tools to protecting yourself and those around you.

For more information about COVID-19 testing, visit henryford.com/testing.

Henry Ford offers COVID-19 vaccines and boosters to established patients. Appointments can be scheduled in MyChart. For updates on booster guidelines and availability of vaccines by age group, visit henryford.com/coronavirus/vaccine-faqs.  

Dr. Dennis Cunningham is the medical director of infection control and prevention at Henry Ford Health.