What Is the Difference Between a PCR Nasal Swab and a COVID-19 Antigen Test?

Medically Reviewed on 5/7/2021
PCR test vs. antigen test
A PCR test is usually more accurate than an antigen test

Both the PCR test and antigen test can be used to determine whether you have been infected with the COVID-19 virus. While it takes longer to get results, a PCR test is usually more accurate than an antigen test.

Generally, you may be tested for COVID-19 if you:

  • Have any symptoms of COVID-19.
  • Have been in contact with a confirmed COVID-19 case, meaning you were within six feet for 15 minutes or more of someone who tested positive for COVID-19.

How does the PCR test work?

The PCR test looks for the presence of the COVID-19 virus by detecting its genetic material (RNA) through a technique called reverse transcriptase-polymerase chain reaction (RT-PCR).  Samples that can be collected for this test include:

  • Nasal swab
  • Throat swab
  • Saliva sample

If the sample contains the virus, its RNA will be extracted. The RNA is converted to DNA and made into several copies. These DNA copies are then detected by a machine. 

The PCR test is currently the most accurate test for diagnosing COVID-19 virus infection and has a low risk of false-negative results (a false-negative occurs when you get a negative PCR result even though you are infected with the virus). 

The accuracy of the PCR test, like other viral tests, depends on factors such as:

  • Technique used to collect the sample
  • Stage of the illness when the sample was collected
  • Whether the sample was transported to the lab under appropriate conditions

One drawback to this test is that results may come back positive even after you have recovered from the infection. This is because the test detects even a small amount of viral RNA even when you are longer infected there is no live virus. 

PCR test results may be available within 24 hours, but it may take longer if the lab is located far away from the sample collection site or if the lab has a large sample load.

How does the antigen test work?

The antigen test detects bits of certain proteins, called viral antigens, present on the surface of COVID-19 virus. Samples are taken from nasal swabs. 

This test is also referred to as a “rapid test” because results may be available much faster (within 15-60 minutes). One advantage of that is that it can allow for more immediate diagnosis and treatment if you test positive, since the antigen test is highly specific for COVID-19.

However, antigen tests have a high rate of false-negative results and are not as accurate as PCR tests. Since a negative antigen test result does not necessarily mean that you are not infected with the virus, your doctor may advise a PCR test to confirm the result.

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Medically Reviewed on 5/7/2021
References
https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/testing.html

https://www.emedicinehealth.com/pcr_test_nasal_swab_vs_antigen_test_covid-19/article_em.htm