Dear Editor,
There seems to be low confidence in the use of antigen tests to help detect SARS-CoV-2, and I would like to advise those who think as such, not to dismiss these tests just like that; they are not perfect but a good aid in diagnostics and do have advantages (discussed later). First, the SARS-CoV-2 virus particle structure comprises infectious genetic material encased in a shell (capsid) made of proteins and fats. Current testing for this virus is based on detecting specific signature pieces of its genetic material, the proteins (antigens) or antibodies specific to SARS-CoV-2 generated in the im-mune response after infection. There are many tests available and many in development where researchers continue to make improvements. The three main types are:
1. Polymerase Chain Reaction (PCR) Test
This is the main test used so far by most countries and the one talked about in the news all the time. It detects signature pieces of the virus’ genome and is reliable and sensitive because super tiny amounts can be amplified and detected, making detection of trace amounts of the virus’s genetic material possible. So even if you have small amounts of virus in your system it is likely to detect it.
2. Serology Test
This is a powerful test that can tell whether you’ve been infected with a virus in the past as it looks for specific SARS-CoV-2 antibodies produced after an immune response (when you get infected). It can also detect specific antibodies after vaccination to confirm that vaccines are successful at generating neutralizing COVID antibodies to fight the virus. It’s not a test used in general diagnostics, but to figure out how a virus spreads in a population, and how long immunity lasts after infection by searching for these specific antibodies.
3. Antigen Test
These tests use lateral flow similar to the at-home pregnancy tests and detect viral proteins (not the genetic material) of SARS-CoV-2. You get a result quickly (within 15 minutes) compared to PCR which takes at least a day, but a drawback (and the main reason there is low confidence in this test) is that the viral proteins can’t be amplified like viral genetic material (in the PCR test) so it’s harder to detect these viral proteins if you’re not carrying a lot of viral particles (low viral load) like in the early stage of an infection.
I’ve been fortunate to have the use of rapid antigen tests on regular basis early on the pandemic for in-person work purposes in Quebec (Canada) and now millions of these tests are being rolled out across Canada (for at-home use) as PCR testing capacity struggles in this Omicron driven wave. In Quebec, the testing capacity is 30,000 PCR tests per day, but at the peak of the wave (within the past couple of weeks) when there were more than 10,000 COVID cases per day, 60,000 people were showing up at testing centres. They were overwhelmed with the province then moving to PCR testing, only for priority groups, and making rapid antigen tests more accessible to the general population (a country-wide strategy right now in Canada with health systems, including testing centres, under severe stress in the Omicron driven wave).
PCR tests are superior in sensitivity but we can still get the best out of antigen tests if published guidance is followed. Antigen tests work best and are a good tool when the viral load (lots of viral particles in our system) is high, as when someone is very infectious. As a person contracts the virus there is an incubation period (several days) where the viral particles build up. The PCR test can detect an infection up to 24 hours earlier than an antigen test, but the antigen test will give a quick result and I see this as an advantage if you test positive (with an antigen test); you can isolate more quickly than if you have to wait several days for the PCR test results, and in the meanwhile, walking around infecting others (if you have COVID). They are also easy to use for oneself, not requiring specialist technical training to perform like PCR experiments. They work best if people are testing themselves on a regular basis (ideally at home).
The US has been slow in adopting antigen tests on a large scale because of their lower sensitivity than PCR tests, however, there are many American scientists who are advocating for these tests as an added tool as infection numbers surge. Many argue, and have shown, that although antigen tests are lower in sensitivity, if testing is done frequently, in spite of its lower sensitivity, it’s a very effective filter in separating the infectious from the non-infectious (quickly). Some further argue that if done on a large scale, systematically, it can actually be used to drive down transmission in local communities and schools and, can be a good strategy for containment (Mina, et al., N Engl J Med 2020; 383:e120DOI: 10.1056/NEJMp2025631).
Science has provided, and continues to provide, many tools (none perfect) to help deal with the pandemic, but to get the most out of them, we need to use them according to best practises (while being aware of any limitations). I think the turnaround time for COVID diagnosis is priority over testing sensitivity, particularly in high-risk environments like when there are major infection surges like this Omicron driven wave. Further, in these circumstances, antigen tests used on a large scale and regularly, can be good filters in separating the highly infectious from others; of course, if one’s situation is severe and the antigen test result is still negative, the general guidance is to get the PCR test. We just need to remember, for antigen tests, to think in context of when in the course of an infection they work best. Ideally, testing continually as symptoms progress is best.
Finally, there are many countermeasures in this pandemic (no single one being perfect) where each adds an extra layer of protection against COVID but vaccination remains at the forefront as global data overwhelmingly shows most people who experience the worse form of the disease, are unvaccinated. So, if you haven’t as yet, get vaccinated and get boosted.
Sincerely,
Jacquelyn Jhingree, PhD.
(Scientist in Vaccine/Drug R&D)