Photo: © Fabio Balbi/Stock.adobe.com
Molecular biologist Nathaniel Hafer from the University of Massachusetts broke down what you need to know about the different Covid-19 tests and how they work.
A version of this article was originally published by The Conversation (CC BY-ND 4.0)
At this stage of the pandemic, you or someone you know may have been tested for Covid-19 at least once. But do you know what kind of tests you have undergone and the advantages and disadvantages of these different tests?
I am a molecular biologist. Since April 2020, I have been a team member of the National Institutes of Health funded project RADx, which is helping innovators develop rapid tests to detect when a person is infected with SARS -CoV- 2. The virus that causes Covid-19.
Two main types of tests are used to diagnose SARS-CoV-2 infection: molecular tests (also called PCR tests) and antigen tests. Each detector detects different parts of the virus, and the way it works will affect the speed and relative accuracy of the test. So what is the difference between these types of tests?
The first step in any test is to obtain a sample from the patient. This can be a nasal swab or a bit of saliva.
For PCR testing, the next step is to amplify genetic material so that even a small amount of coronavirus genes in patient samples can be detected. This is done using a technique called polymerase chain reaction (PCR).
A medical worker takes a sample and processes 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 to separate the DNA into two single-stranded DNA fragments. When the temperature drops, the polymerase binds to the single-stranded DNA and replicates it with the help of a small piece of guide DNA called a primer. The primers ensure that only coronavirus DNA is amplified. You have now created two copies of coronavirus DNA from the original piece of RNA.
The laboratory machine repeats these heating and cooling cycles 30 to 40 times, doubling the DNA until there are 1 billion copies of the original fragment. The amplified sequence contains a fluorescent dye that can be read by a machine.
The amplification properties of PCR allow the test to successfully detect even the smallest amount of coronavirus genetic material in the sample. This makes it a highly sensitive and accurate test. Its accuracy is close to 100pc, which is the gold standard for diagnosing SARS-CoV-2.
However, PCR testing also has some weaknesses. They require skilled laboratory technicians and special equipment to run them, and the amplification process may take an hour or more from start to finish. Usually only large, centralized testing facilities (such as hospital laboratories) can perform multiple PCR tests at once.
Fast and accurate testing is essential to contain highly infectious viruses such as SARS-CoV-2. The PCR test is accurate, but it may take a long time to produce results. Antigen test is another major coronavirus test. Although it is much faster, it is less accurate.
Antigens are substances that cause the body to produce an immune response-they trigger the production of antibodies. These tests use laboratory-made antibodies to search for antigens of the SARS-CoV-2 virus.
To perform an antigen test, you first need to treat the sample with a liquid containing salt and soap to break down cells and other particles. Then apply this liquid on a test paper with SARS-CoV-2 specific antibodies drawn with thin lines.
Just like the antibodies in your body, the antibodies on the test strip will bind to any antigen in the sample. If the antibody binds to the coronavirus antigen, a colored line will appear on the test strip, indicating the presence of SARS-CoV-2.
Stock image of a person using an antigen test. Photo: © Gajus/Stock.adobe.com
Antigen testing has many advantages. First of all, they are very easy to use, even at home, people without special training can execute them and interpret the results. They can also produce results quickly, usually in less than 15 minutes. Another benefit is that these tests can be relatively cheap.
Antigen testing does have some disadvantages. Depending on the situation, they may not be as accurate as PCR tests. When a person has symptoms or a large number of viruses in the system, the antigen test is very accurate. However, unlike molecular PCR tests, antigen tests will not amplify what they are looking for. This means that there needs to be enough viral antigen in the sample for the antibody 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 where the sample is taken. Therefore, the antigen test may miss the early cases of Covid-19. It is also at this stage that a person has no symptoms, so they are more likely to be unaware that they are infected.
In the RADx project that I participated in, we are currently conducting clinical research to better understand the performance of antigen testing at various stages of infection. The more data scientists know about how accuracy changes over time, the more efficiently these tests can be used.
Understanding the advantages and limitations of PCR and antigen testing, and when to use them, will help control the Covid-19 pandemic.
Nathaniel Hafer is an assistant professor of molecular medicine at the University of Massachusetts Chen School of Medicine. He also serves as the Director of Operations of the Center for Clinical and Translational Science at the University of Massachusetts.
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