Molecular biologist explains the difference between a PCR and antigen Covid-19 test

Two major types of tests are used to diagnose infection with SARS-CoV-2: molecular tests—better known as PCR tests—and antigen tests. Each detects a different part of the virus, and how it works influences the test’s speed and relative accuracy. What are the differences between these types of tests?

PCR tests

The first step for either kind of test is to get a sample from the patient. This can be a nasal swab or a bit of saliva.

For PCR tests, the next step is amplification of genetic material so that even a small amount of coronavirus genes in the patient’s sample can be detected. This is done using a technique called a polymerase chain reaction. The goal is to create two copies of coronavirus DNA from the original one piece of RNA.

The amplifying property of PCR allows the test to successfully detect even the smallest amount of coronavirus genetic material in a sample. This makes it a highly sensitive and accurate test. With accuracy that approaches 100 percent, it is the gold standard for diagnosing SARS–CoV–2.

However, PCR tests have some weaknesses too. They require a skilled laboratory technician and special equipment to run them, and the amplification process can take an hour or more from start to finish. Usually only large, centralized testing facilities—like hospital labs—can conduct many PCR tests at a time. Between sample collection, transportation, amplification, detection and reporting, it can take from 12 hours to five days for a person to get results back. And finally, they aren’t cheap at $100 or more per test.

Antigen tests

Antigen tests, 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. 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 minute, and can be relatively inexpensive at around $10-$15 per test.

Depending on the situation, however, 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. But 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.

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. The next time you get a Covid-19 test, choose the one that is right for you. The Conversation


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