The tests to detect viruses are an essential tool in a health emergency situation to control covid-19. Each type of test has its function and utility. Let’s see what antigens are like and why they can be a very useful tool for coronavirus control.
How does an antigen test work?
These tests confirm the presence of the virus by detecting its proteins or antigens. There are different techniques or supports, but more or less all have the same foundation.
Take for example a chromatographic test. It consists of taking a sample from the nose with a swab or saliva, adding a few drops of a reagent that extracts the virus proteins, and mixing well for one or two minutes. Then, with a small plastic pipette, place two or three drops of the sample in the window “S” (sample) from the device, and wait another 15 minutes.
In the “S” window there is a small sponge that is soaked with the sample and reagents. The mixture moves by capillary action through the paper strip. The two key components in this case are a protein labeled with green polyester latex, which is used as a control to check that the system works well, and a monoclonal antibody against the glycoprotein on the surface of the virus envelope (protein S , spike), marked with red polyester latex.
In another area of the device there is a window with the letters “C” (for “control”, not for coronavirus) and “T” (test). In “C” there are antibodies that will bind specifically with the control protein, so that it will be captured by the antibodies and will remain fixed at that level, giving rise to an antibody-protein reaction that will appear as a green line (remember that tiny green latex balls were attached to the protein). The appearance of a green line at the level of “C” therefore means that the control has gone well, that the test reagents have worked correctly.
On the other hand, in the area of the device where the “T” appears, there are specific antibodies against the S protein of the virus. If it turns out that there were viral particles in the sample, they will bind to the red polyester latex-labeled antibodies. The virus protein-labeled antibody complex will move by capillary action until it meets the other antibody against the virus protein at the “T” level. This will form an antibody-antigen-labeled antibody sandwich complex, which will give rise to a red line at the “T” level.
On the contrary, if there was no virus in the sample, this reaction will not occur and no band will appear at the “T” level.
In conclusion, the results that we can obtain are:
No band appears, neither in “C” nor in “T”: the control has not worked well, then the test is invalid. We cannot conclude any results, neither negative nor positive.
Green color band in “C” and nothing in “T”: the control works well and virus antigens are not detected, result negative.
Green band in “C” and red band in “T”: the control works well and virus antigens have been detected, result positive.
The green band does not appear in “C”, but the red band does appear in “T”: the control has not worked well, then the test is also invalid. Nor can we conclude any result, neither negative nor positive, even if the red band has appeared in “T”.
How to interpret a negative result
If it has been negative, does it mean that we are not infected? No. In most cases, from the beginning of the infection, there are a few days (four or five) in which the amount of virus in the sample may be very small and insufficient for the antigen test to detect.
The antigen detection limit is higher than the RNA detection limit by PCR: a higher viral load is required for the antigen test to detect the virus. Therefore, it can happen that the antigen test is negative but that you are in the first days of the infection, and even highly contagious.
In other words, a negative test is not a guarantee of not being infected and of not being able to infect others.
How to interpret a positive result
If we have tested positive, what does it mean and what should we do? It means, with a very high probability, that we are infected with the virus. Our viral load is high enough to be detected by the test. We will also be in the most contagious phase, and we can transmit the disease very easily even if we do not present symptoms. We should isolate ourselves immediately, notify our health center to confirm the result by means of a PCR (a much more sensitive test) and notify our contacts.
As we have just seen, the advantages of the antigen test are its speed and simplicity. It does not require expensive reagents, machines or highly qualified technical personnel, and it is much cheaper than PCR. To top it all, you can do it yourself at home.
Although rapid antigen tests have been the subject of some controversy due to their low sensitivity compared to PCR, there is a growing body of data demonstrating their enormous potential to control the pandemic.
Why can antigen tests help control the pandemic?
The detection of SARS-CoV-2 is essential for the control of the disease and has been based mainly on quantitative RT-PCR (polymerase chain reaction, with reverse transcriptase).
This PCR detects the virus genome from nasal or pharyngeal samples and amplifies it in 30 to 40 cycles, allowing even a minimal number of copies of the virus RNA to be identified. PCR is a very powerful clinical test, especially when a patient is or was recently infected with SARS-CoV-2. The virus’s RNA fragments can remain for weeks after the infectious virus has been shed, often in people without symptoms.
However, another approach is needed for public health measures. The objective is not to know if someone has RNA from the virus in their nose, perhaps from a previous infection, but if they are infectious at this specific moment.
Most people infected with SARS-CoV-2 are contagious for 4 to 8 days. Specimens generally do not contain potentially contagious virus (culture positive) beyond day 9 after onset of symptoms, and most transmission occurs before day 5. This time is consistent with observed patterns of transmission of the virus (usually two days before to 5-7 days after the onset of symptoms). That’s where the recommendation of 10 days of isolation comes from.
This transmissibility window contrasts with a mean of 22–33 days in which CRP can be positive (longer with severe infections and somewhat shorter among asymptomatic individuals). This suggests that 50-75% of the time that an individual is PCR positive they are likely to be post-infectious (traces of the virus RNA are detected but the virus is not active).
PCR amplifies, antigen test does not
Once the immune system has controlled the replication of SARS-CoV-2, the levels of PCR-detectable RNA in respiratory secretions drop to very low levels when individuals are much less likely to infect others. The remaining RNA copies can take weeks, even occasionally months, to disappear, during which time the PCR remains positive.
It therefore makes sense that a test such as antigen has a sensitivity of 30-40% compared to PCR when testing a random sample of asymptomatic people.
Although antigen tests have a lower analytical sensitivity than PCR (they require more virus to test positive), their ability to detect individuals with a high viral load and therefore contagious is as high as CRP. And its specificity, that is, the ability to correctly identify those who are not infected, is comparable to PCR.
This image summarizes a work that compares the result of the antigen test, the PCR (the Ct value or cycle in which it is positive) and the culture, which allows detecting the viability of the virus, in asymptomatic people. It shows that the antigen test is capable of detecting 100% of the PCR positive samples with a positive culture, those with the highest viral load (lowest Ct) and the most contagious (because the virus is active: they are culture positive).
If what we want is to identify a patient who is infected by the virus, to establish whether or not they are sick in people with or without symptoms, we need a very sensitive test (not false negatives) and specific (not false positives). ). In this case, the test of choice would undoubtedly be PCR.
But if what we are looking for is to quickly detect the most contagious people (with or are symptoms, but with more viruses) to avoid transmission, PCR (an expensive test that takes an average of 24-48 hours) may not be the best option. It is preferable to opt for the antigen test.
Yes, it is less sensitive than PCR, but its low sensitivity is compensated by the frequency of use, as it is fast, simple and cheap. Furthermore, it has recently been confirmed that the detection of SARS-CoV-2 in saliva has a high concordance rate with that of nasopharyngeal samples. And what is more important: only 2% of individuals carry 90% of the virions that circulate within communities (super-carriers and possibly also super-contagioners). That is why it is so important to detect them in time.
What is better, a test with 100% sensitivity such as PCR or an antigen test with a lower sensitivity but much faster and that we can repeat frequently?
To detect contagious, a quick and frequent test
In addition to antigen tests, there are already other types of technologies that allow the development of rapid tests that could be used at home. For example, so-called isothermal amplification (LAMP) loop-mediated isothermal amplification) is based on molecular techniques. And, unlike PCR, it requires only a constant temperature for the amplification reaction and identification of a fragment of the virus’ genetic material.
This test is much faster than PCR (less than 30 minutes) and very sensitive. There are already several companies that market this technology in the form of portable kits. In November 2020, the FDA approved the first self-test for home use based on this technique, called Lucira. Although it is not yet commercially available and a prescription will be needed for its use.
Vaccinate, vaccinate, vaccinate and test, test, test
Pregnancy tests, ovulation tests, HIV self-tests, and test strips for glucose level detection are already sold in pharmacies (without a prescription); Covid antibody test, celiac test, urine infection strips (with prescription), and insulin, heparin and glucagon auto-injectables (with prescription). Not only vaccines, but also household antigen tests for SARS-CoV-2 could turn the tide of the pandemic.
It is a matter of time, but they will end up on the market. My bet is to flood the market with self-tests of antigens, to do many and very frequently. That they are sold anywhere, as is already the case in other countries, and that we can do them frequently, several a week to monitor access to schools, universities, health centers, places of leisure, meetings, residences.
If it comes out negative, as we have already said, it does not mean that you cannot infect. But if it comes out positive, immediate isolation.
The original version of this article was published on the author’s blog, microBIO.