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At this point in the pandemic, you or someone you know has probably received at least one COVID-19 test. But do you know which kind of test you got and the strengths and weaknesses of these different tests?

All COVID-19 tests start with a sample, but the scientific process goes differently after that.

, and since April 2020 I’ve been part of a team that is helping innovators develop rapid tests to detect when a person is infected with SARS-CoV-2, the virus that causes COVID-19.

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

Looking for genetic evidence

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 . A health care worker 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, causing the DNA to separate into two single-stranded DNA pieces. The temperature is lowered, and polymerase, with the help of 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’ve now created two copies of coronavirus DNA from the original one piece of RNA.

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.

Laboratory machines , doubling the DNA until there are a . The amplified sequence contains fluorescent dye that is read by a machine.

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 . With , 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 . 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 . And finally, they aren’t cheap at .

Antigen tests

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 .

Just like antibodies in your body, the ones on the test strip will . 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 – . They also produce results quickly, . Another benefit is that these tests can be relatively inexpensive at .

Antigen tests do have some drawbacks. Depending on the situation, they can be . When a person is symptomatic or has a lot of virus in their system, . 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 . It’s also , so they are more likely to be unaware they’re infected.

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.

More tests, better knowledge

A few antigen tests are already available over the counter, and on Oct. 4, the Food and Drug Administration . The U.S. government is also pushing to .

At RADx, the project I am a part of, we are 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.

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