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Jan 10, 2022
3 min read

Antigen vs. antibody tests: what’s the difference?

Antigen tests (also known as rapid tests) are diagnostic tests that are used to see if you are currently infected with COVID-19. Antibody tests, on the other hand, look for your body’s response to the virus or vaccine and are used to check your immunity to the virus or to see if you were infected in the past.

Important

Information about the novel coronavirus (the virus that causes COVID-19) is constantly evolving. We will refresh our novel coronavirus content periodically based on newly published peer-reviewed findings to which we have access. For the most reliable and up-to-date information, please visit the CDC website or the WHO’s advice for the public.

We’ve all gotten more of an education on infectious disease than we bargained for since the pandemic started. And while we’re almost pros when it comes to understanding which symptoms to look out for, how variants develop, and what the vaccine actually does to protect you from COVID-19, words like “antigen” and “antibody” can leave us scratching our heads. 

To put it simply: antigen tests (also known as rapid tests) look for the virus in your system and check to see if you’re infected with COVID-19 right now. Antibody tests look for your body’s reaction to the virus (think: antibody = your body’s response). There’s also a difference between how the tests are collected: antigen tests are usually a quick nasal swab while antibody tests require a blood sample either from a finger stick or a vein. 

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When should I take an antigen test?

Antigen tests are available both in-person and at home and can be used if you have symptoms of COVID or if you were recently exposed to a person who tested positive for coronavirus. The tests usually take a minute or two to do and results are available within about 15 minutes. 

If you were exposed to a person who has tested positive for the coronavirus, the CDC recommends waiting 5–7 days after you were exposed before you get tested, as your results are unlikely to be positive before then. Even if you do have the virus, a negative test before 5 days won’t mean that you aren’t infected (CDC, 2022). 

If you have symptoms, it’s a good idea to get tested and try to steer clear of the other members of your household if possible. Symptoms of COVID-19 include: 

  • Fever and chills
  • Cough
  • Shortness of breath, difficulty breathing
  • Fatigue
  • Muscle or body aches
  • Headache
  • Sore throat
  • A loss of taste or smell
  • Congestion or runny nose
  • Nausea, vomiting, and diarrhea

When should I get an antibody test?

Antibody tests look for your immune system’s reaction to the virus from previous infection or vaccination. When your immune system is exposed to the virus (or some component of it, in the case of vaccination), it produces small molecules that act like magnets which can selectively stick to cells in your body infected with that particular virus. They then help destroy these infected cells, preventing the infection from spreading (Chen, 2005). 

Infection causes these antibodies to be generated, as does vaccination. Theoretically, antibodies should be an indication of protection against future infection. Unfortunately, though, the picture is a little more complex. There are different types of antibodies and having a positive antibody test is not a guarantee of immunity. 

While you might see antibody tests available at your local urgent care clinic or pharmacy, researchers aren’t yet sure whether a positive antibody test indicates real protection against future infection with coronavirus or its different variants, making current antibody testing somewhat less useful (Jiang, 2020).

What about PCR tests?

Like antigen tests, PCR tests are diagnostic tests, meaning they can be used to determine if you have the virus in your body right now. But, as opposed to antigen tests, which are pretty fast and can be done entirely at home, PCR tests require special equipment so the results take longer. 

And while PCR tests are a little bit more accurate when it comes to detecting the virus, that’s not always a good thing. PCR tests can remain positive for months after a person is no longer at risk of transmitting the virus to others, meaning they aren’t a good method for deciding whether or not you should quarantine (Sule, 2020; Sidiq, 2020). 

Comparison of different tests

References

  1. Centers for Disease Control and Prevention — COVID-19 Testing Overview. (2021-a, August 2). Retrieved September 9, 2021 from https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/testing.html
  2. Centers for Disease Control and Prevention — Interim Guidelines for COVID-19 Antibody Testing. (2021-b, Sept 21). Retrieved Jan. 11, 2022 from https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html#anchor_1590264273029
  3. Chen, S., Lu, D., Zhang, M., Che, J., Yin, Z., Zhang, S., et al. (2005). Double-antigen sandwich ELISA for detection of antibodies to SARS-associated coronavirus in human serum. European Journal of Clinical Microbiology & Infectious Disease; 24(8): 549-53. Retrieved from https://pubmed.ncbi.nlm.nih.gov/16133409/
  4. Chorba, T. (2020). The Concept of the Crown and Its Potential Role in the Downfall of Coronavirus. Emerging Infectious Diseases, 26(9), 2302-2305. Retrieved from https://pubmed.ncbi.nlm.nih.gov/32997903/
  5. Jiang, J. C., & Zhang, Y. (2020). Serological antibody testing in the COVID-19 pandemic: their molecular basis and applications. Biochemical Society transactions, 48(6), 2851–2863. doi: 10.1042/BST20200744. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/33170924/
  6. Sule, W. F., & Oluwayelu, D. O. (2020). Real-time RT-PCR for COVID-19 diagnosis: challenges and prospects. The Pan African Medical Journal, 35(Suppl 2), 121. doi: 10.11604/pamj.supp.2020.35.24258. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33282076/
  7. Sidiq, Z., Hanif, M., Dwivedi, K. K., & Chopra, K. K. (2020). Benefits and limitations of serological assays in COVID-19 infection. The Indian Journal of Tuberculosis, 67(4S), S163–S166. doi: 10.1016/j.ijtb.2020.07.034. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33308664/