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Tests for COVID-19 are effective, but come with limitations.
Image by Markus Spiske on Unsplash
Although almost every individual on the planet is familiar with COVID-19, there are many misconceptions about how coronavirus tests work. There are two main tests related to the virus that causes COVID-19; we will explore what they detect, how detection works, and what these tests can tell us about the infection.
Diagnosing Patients Using RT-PCR Nasal Swab Tests
Nasal swabs are used to collect samples from the mucus membranes in the patient’s nose and throat, which are then sent to a lab for testing. A RT-PCR assay is used to detect the virus’s genetic material, RNA, which is extracted and purified. The RNA is then converted to DNA and mixed with primers.
Cycles of heating and cooling the primers with viral DNA, along with a DNA-building enzyme, results in millions of copies of virus DNA. If the sample contains viral DNA, fluorescent dye molecules cause the sample to glow, indicating a positive result. Otherwise, the sample remains dim, which is indicative of a negative result.
A nasal swab test is a critical tool in determining which individuals should quarantine to help combat the spread of COVID-19. However, it does not show whether the RNA is part of an infectious virus particle.
Testing Patients for Exposure to COVID-19
Blood tests, antibody tests, and serology tests are used to test for an immune response to the SARS-CoV-2 virus that causes COVID-19. These tests use blood samples to detect proteins known as antibodies formed in our bodies in response to an infection, such as with coronavirus. Antibodies are found in blood serum, a transparent liquid that is left over once red blood cells and proteins are removed.
ELISA is most commonly used technique to detect these antibodies. In this technique, lab plates are coated with samples of SARS-CoV-2 proteins. The serum sample, along with a detector antibody, is added to the ELISA plate. If the serum contains antibodies developed due to COVID-19, the antibodies bind to the viral proteins coating the ELISA plate.
The detector antibody is coupled with an enzyme and can only bind to the patient’s antibody. If the patient’s serum sample contains antibodies which bind to the coronavirus proteins, the enzyme is activated causing a optical color change, which can be detected by highly specialized ELISA plate reader. The change in color is indicative of an immune response to coronavirus (due to the presence of IgM/IgG antibodies) and explained as COVID-19 positive.
Serology tests are poor indicators of active infection as antibodies to viruses linger after the disease has passed. In fact, most serology tests are better at showing prior exposure as they are designed to detect a specific type of antibody (IgM/IgG) that appear later on in the infection.
These tests, however, cannot show whether an individual is protected from reinfection by COVID-19.
How Scientists Use Coronavirus Tests
Tests that detect both active infection and prior exposure are vital in understanding the scope of the COVID-19 pandemic. Serology tests are also used in studies for the potential therapeutic use of antibodies from patients who have recovered from COVID-19.
As countries reopen their economies and companies race to develop a vaccine, new tests must also be considered. Neither the swab test, nor the serology test, indicates whether an immune response to COVID-19 prevents reinfection. This is vital information which currently available diagnostics tests cannot provide.
Research and development in this are is at an all time high. There are countless diagnostics tests being developed my pharmaceutical and biotech companies worldwide. Only time will tell which of these will have the potential to address these key diagnostic issues and improve public health outcomes in light of the COVID-19 pandemic.
