After two years living in a pandemic we all have probably done at least one COVID-19 test. Although at the beginning molecular tests were most common (you can read more about this topic here), nowadays, rapid tests are used for most of the cases. This post covers the advantages and disadvantages of these tests, as well as when it is more appropriate to use them.
Antigen tests
Antigen tests or rapid tests work by detecting viral proteins within a sample. To further understand the difference between nucleic acids and proteins, you can check here.
For antigen tests, swabs taken in the nose or throat are treated with a reagent, and then placed on a test strip or cartridge. The first area the sample reaches is called the reaction zone. In this area there are antibodies* specific for the SARS-CoV-2 spike protein that will bind to them if there are any. These antibodies also have an enzyme attached to them that will play a part further along the strip. Shortly after, the test zone includes another type of antibodies attached and unable to move. These will also bind to the spike protein if there is any, creating a sandwich. The enzyme will trigger a colour change and a line will appear. If there is no spike protein, the mobile antibodies will just sail past [1]. Within 30 minutes, the results of the test are obtained. This is the same principle as with home pregnancy tests!
Whether the test is positive or negative, another line should appear on the control zone of the strip. Here, more immobile antibodies will bind to the excess of mobile antibodies that didn’t pick up the spike protein, and will trigger a colour change. This proves the test is functioning correctly [1].
What are the advantages?
Cheap and easily accessible.
Results are obtained rapidly.
No specialized laboratory equipment required.
Useful to test whether one is still infectious, after recovering from COVID-19.
What are the disadvantages?
Less sensitive than molecular tests, as sensitivity depends on the stage of the infection.
A high chance of false negatives.
Typically require PCR test to confirm result.
When should these tests be used?
Ideally after a COVID-19 infection, as this test is useful to deduce when one is no longer infectious. In addition, upon the onset of COVID-19 symptoms, a rapid test can be used to diagnose whether an infection is present. However, if the test is negative, then a PCR test is recommended to confirm the negative result [2].
In terms of diagnosing COVID-19 infections, unfortunately “one size does not fit all”. Each of the two diagnostic tests have contexts in which they are superior and more preferred to their counterpart. In addition, as time goes by and new variants have emerged, the manner in which we sample and process them for these tests have also adapted to fit the dynamic situation. The pandemic has taught us that one of the best tools in preventing the spread of COVID-19, is by regularly testing and isolating positive cases. Consequently, these types of tests will continue to become even more familiar to the average person, and will continue to be one of our best weapons until the pandemic is officially over.
*Antibodies - a molecule produced by immune cells that recognizes and binds to another molecule that is foreign to the organism, the antigen. By binding to the antigen, the antibody can tag the foreign molecule for destruction or directly neutralizes it.
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Written by: Renard
Edited by: María and Natasha
BioDecoded is a volunteer group committed to sharing accurate scientific information. For more information about vaccines and their safety profile, please see previous posts or consult with your personal physician. If you have any questions about this topic, please comment or send them to our email.
References:
Overview of ELISA Testing for COVID-19 Antibodies (2020). Available at: https://www.news-medical.net/whitepaper/20200616/Overview-of-ELISA-Testing-for-COVID-19-Antibodies.aspx (Accessed: 22 March 2022).
When you should take a PCR vs. a rapid antigen test, CNN (2022). Available at: https://edition.cnn.com/2022/01/19/health/pcr-vs-antigen-tests-covid-19-wellness/index.html(Accessed: 22 March 2022).
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