COVID-19

COVID-19 Test Differences - PCR, Rapid, and Antibody

Jacqueline Slobin08 Sep 2021

There are currently two main tests for the COVID-19 virus in the United States: diagnostic and antibody tests. 

  • Diagnostic tests detect an active infection with COVID-19 and are either done through a nasal swab or a saliva sample. The two types of diagnostic tests are molecular PCR tests and antigen (rapid) tests. 
  • Antibody tests are done through a blood test to detect a previous infection with COVID-19. 

The Differences Between a COVID-19 PCR & Antibody Test 

Both PCR and antigen tests are diagnostic tests; therefore, you should get either of these tests if you are experiencing symptoms of COVID-19. According to Harvard Health Publishing, there are a few key differences between the PCR and antigen (rapid) tests. We outline important differences in the table below:

PCR TestAntigen (rapid) Test
Nasal/throat swab or saliva sampleNasal swab
Results can take up to a week Results in less than one hour
Lower chance of false-negative resultsHigher chance of false-negative results 
Works by directly detecting the presence of the virus genetic material (RNA)Directly detects molecules on the surface of the virus

Antigen tests produce results faster, but have a higher chance of yielding false-negative results. Your doctor may want to get a PCR test to confirm a negative test result from an antigen test. 

COVID-19 PCR Test

RT-PCR, or real-time polymerase chain reaction, is used to detect an active infection of COVID-19. Samples for a PCR test can be taken through a nasopharyngeal swab or a saliva test. 

The saliva sample is a less invasive test than taking a nasopharyngeal sample. Like the nasal swab, the saliva test detects active COVID-19 infection in patients experiencing symptoms or who may have been exposed to someone with coronavirus. While the saliva tests are less commonly used than nasal swabs, the accuracy of the tests is very similar. 

How it Works

The RT-PCR technique targets a specific segment of the virus's genetic material to detect the presence of COVID-19. RT-PCR works by making thousands of copies of this genetic material to assess if the virus is present qualitatively. The test only detects the virus while the person is currently infected. 

PCR Test Accuracy

It is important to know that your PCR may yield a false negative result, meaning you are infected with the active coronavirus, but your test result comes back negative. False-negative results may be caused by taking the sample incorrectly or getting tested too soon after exposure. This means that if your swab test comes back negative, you cannot certainly rule out the chance of having COVID-19.

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If you get a PCR test the day you were exposed to someone with COVID-19,  the likelihood of receiving a false-negative test result is 100%. For the most accurate results, healthcare professionals recommend getting tested about 7 days after exposure or about 3-4 days after symptom onset. Consult your provider for more details on the accuracy of the specific test they are using. 

Most PCR tests take several days (typically around 5 days) to process. Some rapid tests use PCR and can yield results within a few hours. For example, the Abbott ID Now test is a rapid PCR test that is FDA approved. 

Rapid COVID-19 Test

Some urgent care facilities, testing centers, and doctors' offices offer a rapid antigen test to diagnose a current infection with COVID-19. Rapid tests may be useful for individuals who need to return to work as soon as possible and cannot wait for the results of a PCR test.

How it Works

The antigen test works by detecting specific molecules on the surface of the virus. This is the same way that rapid strep throat tests are completed. 

Patients can typically get the results of a rapid COVID test in as quickly as 15 minutes. Rapid tests are not available at as many locations as standard PCR tests, so make sure to call in advance to confirm that this test is available. Additionally, rapid tests may be more expensive and require out of pocket costs.

Rapid Test Accuracy

It is important to note that rapid antigen tests are less accurate than PCR tests; thus, they are more likely to yield a false negative result. For this reason, antigen tests are not sufficient for travel to many states in the U.S.  Make sure to consult with your provider regarding the accuracy of the specific antigen test used.

COVID-19 Antibody Test

The antibody test checks to see if individuals have been previously infected with the COVID-19 virus by looking for a specific antibody in the bloodstream. Antibodies are small proteins that are part of our immune system. They are produced to defend our body against a specific virus, for example, COVID-19.

The blood test looks for the specific antibodies that fight infection from the coronavirus. According to the Centers for Disease Control, it normally takes one to two weeks after someone becomes sick with COVID-19 for their body to make antibodies. 

How it Works

The test checks to see if we have the antibodies for COVID-19. Antibody levels are detected through a test called ELISA, or serological enzyme-linked immunosorbent assay, and ultimately tests for the number of antibodies in the blood for the specific virus.

The test tells us if we already had the virus, but it is not currently used for quick detection like the PCR and antigen tests. Antibody tests can detect previous infection with the virus even after the patient has recovered.

Rapid Test Accuracy

Blood test results are typically available within 1-2 days. The antibody test has 2 major implications. First, the test can be used to identify potential blood donors in individuals who recovered from COVID-19. Blood from COVID-19 survivors can possibly be used to treat current COVID-19 patients. Second, the antibody blood test may give us insight into how long the antibodies last after infection with COVID-19. 

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Note: Testing positive for the COVID-19 antibody test means you have previously been exposed to the virus but do not necessarily confer immunity. 

Choosing a COVID-19 Test

In general, PCR tests are more accurate than rapid tests. Therefore, if you can wait about 5-7 days for your results, you should get the PCR test. If you need results immediately, you should get a rapid test. In addition, rapid tests are not qualified for travel to most states with restrictions. You must get a PCR test within 72 hours of travel if going to a state with a travel restriction. 

Where to Get a COVID-19 Test

Testing protocols, regulations, and locations continue to vary by city during the coronavirus outbreak. If you are experiencing symptoms of COVID-19, you should seek medical attention and inquire about getting a test. See the links below for information on testing near you: 

References:

https://www.fda.gov/media/134922/download

https://www.scientificamerican.com/article/heres-how-coronavirus-tests-work-and-who-offers-them/

https://www.pacificimmunology.com/resources/antibody-introduction/what-is-an-antibody/

https://www.livescience.com/how-coronavirus-tests-work.html

https://www.the-scientist.com/news-opinion/first-saliva-test-for-covid-19-approved-for-emergency-use-by-fda-67416

https://www.health.harvard.edu/diseases-and-conditions/if-youve-been-exposed-to-the-coronavirus