At this point in the pandemic, you or someone you know may have received 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’m a molecular biologist, and since April 2020 I’m part of a team working on a National Institutes of Health-funded program called RADx, which uses innovations to detect if a person has been infected with SARS-CoV. Helping to develop rapid tests. 2, the virus that causes COVID-19.
Two major types of tests are used to diagnose infection with SARS-CoV-2: the molecular test — known as the PCR test — and the antigen test. 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?
In search of genetic evidence
The first step for any type of test is to obtain a sample from the patient. This could be a swollen nose or a slight drool.
For PCR tests, the next step is the amplification of the genetic material so that even small amounts of the coronavirus gene can be detected in the patient’s sample. This is done using a technique called polymerase chain reaction. 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 polymerase and heated, causing the DNA to separate into two single-stranded DNA fragments. The temperature is lowered, and the polymerase, with the help of a small piece of guide DNA called a primer, binds to the single-stranded DNA and copies it. Primers ensure that only the coronavirus DNA is amplified. You have now made two copies of the coronavirus DNA from the original one piece of RNA.
Laboratory machines repeat these heating and cooling cycles 30 to 40 times, doubling the DNA until there are one billion copies of the original fragment. The amplified sequence contains a 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 highly sensitive and accurate test. With an accuracy approaching 100%, it is the gold standard for the diagnosis of SARS-CoV-2.
However, PCR tests also have some weaknesses. Running them requires a skilled laboratory technician and specialized equipment, and the amplification process can take an hour or more from start to finish. Typically only large, centralized testing facilities – such as hospital laboratories – can perform multiple PCR tests at a time. Between sample collection, transportation, amplification, detection and reporting, it can take anywhere from 12 hours to five days for results to be returned to an individual. And lastly, they aren’t cheap at $100 or more per test.
Rapid, accurate tests are essential to prevent highly contagious viruses such as SARS-CoV-2. PCR tests are accurate but results can take a long time to come. Antigen testing, the other major type of coronavirus test, while much faster, is 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 separates cells and other particles. You then apply this liquid to a test strip that contains antibodies specific to SARS-CoV-2, painted in a thin line.
Like the antibodies in your body, the antibodies on the test strip will bind to any antigens in the sample. If the antibodies bind to the coronavirus antigen, a colored line appears on the test strip indicating the presence of SARS-CoV-2.
Antigen tests have several 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 give 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. Antigen tests are very accurate when a person is symptomatic or has a lot of virus in their system. 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 so that the antibodies on the test strip can produce a signal. When a person is in the early stages of infection, there is not much virus in his nose and throat from which samples are taken. Therefore, antigen tests may miss early cases of COVID-19. It is also during this stage that a person has no symptoms, so they are more likely to be unaware of being infected.
[Get the best of The Conversation, every weekend. Sign up for our weekly newsletter.]
More tests, better knowledge
Some antigen tests are already available over the counter, and on October 4, 2021, the Food and Drug Administration granted emergency use authorization for another home antigen test. The US government is also pushing to make these tests more available to the public.
At RADX, the project I am part of, we are currently conducting clinical studies to better understand how antigen tests perform 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 done, choose the one that’s right for you.