Approved for sale in Brazil at the end of January, the self-tests began to be marketed in a period of high Covid-19 cases. Since then, 32 products of the type have been authorized by Anvisa (National Health Surveillance Agency).
Even though they are important, self-tests should not be considered the only devices for diagnosing Covid-19 because they can have a false negative. In addition, part of the population still has doubts about when to opt for other tests, such as RT-PCR.
See below for answers to these and other questions.
Is the self test the same thing as quick tests?
Self-testing and rapid antigen tests use similar technologies that look for parts of the Sars-CoV-2 protein in the person suspected of being infected.
However, Anvisa reiterates that the rapid tests should not be used by laypeople because they may have differences in the guidelines for use and in how to collect the material, requiring the performance of a qualified professional.
In the case of self-tests, Anvisa also states that it is important to observe whether the product has been approved — this information can be consulted on the agency’s official website.
When is self-testing recommended?
The self-test should be used when someone has had contact with a person who has tested positive for Covid or when a person experiences some common symptoms of the disease, such as fever, cough, sore throat and loss of smell or taste.
The instrument, however, is not recommended for cases in which the symptoms are already more critical. According to Anvisa, in situations where the person already has more serious symptoms, such as shortness of breath or oxygen saturation below 95%, it is recommended to seek a health service.
For Raquel Stucchi, an infectious disease specialist and professor at Unicamp (State University of Campinas), the self-test is a product for situations in which the person needs to quickly obtain the answer to a suspected case of Covid.
“You have a meeting and you’re not sure if you have Covid because your nose started to run. So, you do a self-test. If it comes back negative, it means that in the place you go you won’t be transmitting it to other people”, he exemplifies.
On which day, from the onset of symptoms, should the self-test be performed?
Anvisa recommends that the self-test be done between the 1st and 7th day from the onset of symptoms. If you do not have symptoms, the agency says that the self-test should be done from the fifth day of exposure to a person who was infected with Sars-CoV-2.
On the other hand, Stucchi explains that the ideal is that the collection is done between the third and the fifth day, mainly in the vaccinated ones. This is because immunizers can make viral replication difficult, causing the person to have been infected by the virus, but there is still no detection in the nasal region. Therefore, the sensitivity of the self-test may take a little longer.
Is the self-test enough for the diagnosis of Covid?
Anvisa says that the self-test is a screening tool in order to prevent the spread of the virus. So when someone takes a positive self-test, they can quickly isolate themselves before passing it on to others.
For Stucchi, the big advantage is its cost and the fact that the result comes out faster. The infectious disease specialist, however, emphasizes that it is necessary to redo new self-tests if the result of the first one is negative or to look for other types of diagnoses for the disease, such as RT-PCR.
Lorena Barberia, a member of the Covid-19 BR Observatory and professor at the Department of Political Science at USP, says that, in Brazil, it is important to improve the orientation of the population to repeat self-testing.
“Many governments, when distributing self-tests, say that it is important to continue testing to monitor how it is evolving”, says the professor.
Is it possible to have a false negative with a self-test?
The self-test uses a technology that checks whether Sars-CoV-2 proteins are present in the region where the swab is inserted. Therefore, it is possible that someone has become infected, but the virus has not yet been able to replicate on a large scale.
This is why the self-test can have a false negative. It is also for this reason that, when doing a self-test with a negative result, it is necessary to repeat at least one more time within 48 hours to make sure that there is no infection.
Even so, a negative result is useful because it indicates that the person has a low chance of transmission. “A negative self-test serves to say that, at that moment, the person may even have the virus, but it has a low amount. So the risk of transmission becomes very small”, says Stucchi.
The infectologist, however, points out that the person with symptoms of Covid and the negative result of the self-test still needs to continue with measures to avoid transmission, such as the use of N95 masks and social isolation. Repeating the self-test is also required.
When should I choose to do an RT-PCR or antibody test instead of a self-test?
Unlike self-test, RT-PCR has a lower chance of having a false negative result. One of the reasons is that it is done in the laboratory and, therefore, tends to have a more accurate sample collection.
As the self-test is done at home, it may happen that the person does not collect the material properly, reducing the sensitivity of the product. “The person has to insert the cotton swab deep into the nose. When a tear comes out, we say that it is harvesting in the right place”, says Stucchi.
In addition, it is important to carefully observe the self-test manufacturer’s instructions in order to properly collect the sample.
Another type of test for Covid is antibody tests. They look at whether the person’s body has already developed an immune response to Sars-CoV-2. However, these tests are not recommended because it is not yet known whether high antibodies mean prolonged protection from the disease, says Stucchi.
“An antibody test would only be useful to know how many people have already been exposed or who have had some protection against Covid. But it is not useful to say if you are transmitting, if you are sick or to guarantee that you will no longer get sick. [porque existem casos de reinfecção]”, he concludes.
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