PCR tests are extremely accurate but require special lab equipment – like the PCR heating machine seen here – and can take hours or days to perform.
Rapid antigen tests – like the two seen here showing negative results – are much faster and easier to perform but are less sensitive than PCR tests.
Lennardywlee via Wikimedia Commons, CC BY-SA
Antigen tests
Rapid, accurate tests are essential to contain a highly contagious virus like SARS-CoV-2. PCR tests are accurate but can take a long time to produce results. Antigen tests, the other major type of coronavirus test, while much faster, are less accurate.
Antigens are substances that cause the body to produce an immune response – they trigger the generation of antibodies. These tests use lab-made antibodies to search for antigens from the SARS-CoV-2 virus.
To run an antigen test, you first treat a sample with a liquid containing salt and soap that breaks apart cells and other particles. Then you apply this liquid to a test strip that has antibodies specific to SARS-CoV-2 painted on it in a thin line.
Just like antibodies in your body, the ones on the test strip will bind to any antigen in the sample. If the antibodies bind to coronavirus antigens, a colored line appears on the test strip indicating the presence of SARS-CoV-2.
Antigen tests have a number of strengths. First, they are so easy to use that people with no special training can perform them and interpret the results – even at home. They also produce results quickly, typically in less than 15 minutes. Another benefit is that these tests can be relatively inexpensive at around $10-$15 per test.
Antigen tests do have some drawbacks. Depending on the situation, they can be less accurate than PCR tests. When a person is symptomatic or has a lot of virus in their system, antigen tests are very accurate. However, unlike molecular PCR tests, antigen tests don’t amplify the thing they are looking for. This means there needs to be enough viral antigen in the sample for the antibodies on the test strip to generate a signal. When a person is in the early stages of infection, not a lot of virus is in the nose and throat, from which the samples are taken. So, antigen tests can miss early cases of COVID-19. It’s also during this stage that a person has no symptoms, so they are more likely to be unaware they’re infected.
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More tests, better knowledge
A few antigen tests are already available over the counter, and on Oct. 4, 2021, the Food and Drug Administration granted emergency use authorization to another at-home antigen test. The U.S. government is also pushing to make these tests more available to the public.
At RADx, the project I am a part of, we are currently conducting clinical studies to get a better understanding of how antigen tests perform at various stages of infection. The more data scientists have on how accuracy changes over time, the more effectively these tests can be used.
Understanding the strengths and limitations of both PCR and antigen tests, and when to use them, can help to bring the COVID-19 pandemic under control. So the next time you get a COVID-19 test, choose the one that is right for you.
Nathaniel Hafer, Assistant Professor, Program in Molecular Medicine, UMass Chan Medical School
This article is republished from The Conversation under a Creative Commons license. Read the original article.
