Covid-19 Antigen Test Explained
By Yiheng Wang
When you swab your nose to take a COVID-19 test, have you ever wondered the underlying principles behind this rapid antigen test? A COVID-19 rapid antigen test can be done within one minute. Yet, there are many scientific principles behind this test. Most of the COVID-19 test kits we encounter are lateral flow assays, and this article will explain the details of this assay.
When you make a swab in your nose, some of your mucus in your nose gets on the swab, which may or may not contains the covid-19 antigen. Then, your mucus is placed into a liquid, which is a buffer with nothing else. The buffer simply dilutes the mucus, to make sure it’s not so sticky, and ensures that the solution is neutral in pH. After this, the sample solution is transferred to the test strip.
Figure 1: Covid 19 Antigen Test Strips. The left has a negative result, while the right has a positive result.
In a COVID-19 test, the presence of two lines indicates a positive result, while one line indicates a negative result. This testing outcome is caused by the lateral flow assay design of the test strip. When the diluted mucus sample migrates along the test strip, it encounters antibodies conjugated with gold nanoparticles (AuNP), which serve as visible markers that bind specifically to viral antigen if present, enabling the formation of test lines.
You may wonder if the gold nanoparticles are expensive, but they are actually quite affordable because the test strip requires a minimal amount of gold. These trace quantities are sufficient to produce visible color on the test strip. If the sample contains viral antigens, the antibodies conjugated with the nanoparticles will bind to this antigen. The antibody-antigen complexes will migrate with the sample fluid. Upon reaching the test line, which contains fixed antibodies specific to the viral antigen, these fixed antibodies would capture the complex, forming the visible line. Then, the liquid sample and antibody flow to the second line, the control line, which contains some fixed secondary antibody. Secondary antibodies are “antibodies of antibodies”, which can detect the presence of certain antibodies. If the test strip works normally, the secondary antibodies will bind with the moving antibodies with AuNP, so the gold nanoparticles will stop here and eventually form a single line. This control line is crucial as it prevents the occasion of false positives due to test strip problems. With all these measures, we can know whether we get COVID-19.
Figure 2: An Illustration for Lateral Flow Assay
Limitations of Rapid Antigen Assay
Although the assay demonstrates high accuracy, there is still the chance for the antibodies to be “deceived”. A false positive may occur when the antibodies detect fragments of dead COVID-19 virus, or if foreign substances, such as spilled beverage, contaminate the test strip. For example, many newly recovered covid-19 patients may still get their antigen test results positive. The antibodies may also be “misled” into producing a false positive if they bind to molecules or proteins with structures which are similar to the target antigen. However, though there might be chances to have false positive in a rapid antigen test, it is quite rare. In all, taking a more accurate nucleic acid test is a good choice for you to confirm whether you’re truly infected.
