What is the difference between a PCR and antigen COVID-19 test? A molecular biologist explains: – Community News
Covid-19

What is the difference between a PCR and antigen COVID-19 test? A molecular biologist explains:

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(THE CONVERSATION) At this point of the pandemic, you or someone you know has likely undergone at least one COVID-19 test. But do you know what kind of test you got and what are the strengths and weaknesses of these different tests?

I am a molecular biologist and since April 2020 I have been part of a team working on a National Institutes of Health funded program called RADx that is helping innovators develop rapid tests to detect when a person is infected with SARS-CoV – 2, the virus that causes COVID-19.

There are two main types of tests used to diagnose SARS-CoV-2 infection: molecular tests – more commonly known as PCR tests – and antigen tests. Each detects a different part of the virus and how it works affects the speed and relative accuracy of the test. So what are the differences between these types of tests?

Looking for genetic evidence

The first step for both types of tests is to get a sample from the patient. This could be a nasal swab or some saliva.

For PCR testing, the next step is to amplify genetic material so that even a small amount of coronavirus genes can be detected in the patient’s sample. This is done using a technique called a polymerase chain reaction. A healthcare professional takes the sample and treats it with an enzyme that converts RNA into double-stranded DNA. Then the DNA is mixed with a solution containing an enzyme called a polymerase and heated, which separates the DNA into two single-stranded DNA pieces. The temperature is lowered and polymerase, using a small piece of guide DNA called a primer, binds to the single-stranded DNA and copies it. The primers ensure that only coronavirus DNA is amplified. You now have two copies of coronavirus DNA made from the original piece of RNA.

Lab machines repeat these heating and cooling cycles 30 to 40 times, doubling the DNA until there are a billion copies of the original piece. The amplified sequence contains fluorescent dye which is machine read.

Thanks to the amplifying property of PCR, the test can successfully detect even the smallest amount of coronavirus genetic material in a sample. This makes it a very sensitive and accurate test. With an accuracy approaching 100%, it is the gold standard for diagnosing SARS-CoV-2.

However, PCR testing also has some weaknesses. They require a skilled lab technician and special equipment to make them work, and the amplification process can take an hour or more from start to finish. Usually, only large, centralized testing facilities — such as hospital labs — can run many PCR tests at once. Between sample collection, transport, amplification, detection, and reporting, it can take anywhere from 12 hours to five days for a person to get the results back. And lastly, they’re not cheap at $100 or more per test.

Antigen Testing

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 get results. Antigen testing, the other major type of coronavirus test, is much faster, but less accurate.

Antigens are substances that cause the body to produce an immune response – they trigger the production of antibodies. These tests use lab-created antibodies to look for SARS-CoV-2 virus antigens.

To perform an antigen test, first treat a sample with a liquid containing salt and soap that sets cells and other particles apart. You then apply this liquid to a test strip on which antibodies specific for SARS-CoV-2 have been painted in a thin line.

Just like antibodies in your body, those on the test strip will bind to every antigen in the sample. If the antibodies bind to coronavirus antigens, a colored line will appear on the test strip indicating the presence of SARS-CoV-2.

Antigen testing has a number of strengths. First, they are so easy to use that people without special training can perform them and interpret the results – even at home. They also produce results quickly, usually in less than 15 minutes. Another advantage is that these tests can be relatively cheap at around $10-$15 per test.

Antigen tests have some drawbacks. Depending on the situation, they may be less accurate than PCR tests. When a person is symptomatic or has a lot of viruses in their system, antigen tests are very accurate. However, unlike molecular PCR tests, antigen tests do not amplify what they are looking for. This means that there must 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, there is not much virus in the nose and throat, from which the samples are taken. Thus, antigen testing may miss early cases of COVID-19. It is also during this stage that a person has no symptoms, so they are more likely not to know they are infected.

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More testing, better knowledge

A few antigen tests are already available over the counter, and on October 4, 2021, the Food and Drug Administration approved emergency use for another at-home antigen test. The US government is also pushing for these tests to be made more available to the public.

At RADx, the project I’m part of, we’re currently conducting clinical trials to better understand how antigen testing performs at different stages of infection. The more data scientists have about 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 bring the COVID-19 pandemic under control. So the next time you get a COVID-19 test, pick the one that’s right for you.