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After all, what do the tests test?

27
Apr
2020
The genetic material of the virus itself is sought. In other words, these tests detect nucleic acids - in this case, the virus's ribonucleic acid (RNA). For this purpose, in these tests, a technique called “polymerase chain reaction” - or PCR, is used. In fact, the World Health Organization (WHO) and the European Center for Disease Control (ECDC) recommend tests using the PCR methodology to detect SARS-CoV-2, the virus responsible for the covid-19 disease and which has suddenly erupted across the planet from December last year. This is because this type of tests allows to detect the actual presence of the virus at that moment in the person's body. “The patient is only positive if he has a viral load. If you have recovered, the test will not be positive ”, explains Vasco M. Barreto, biology doctorate in immunology at the Center for Disease Studies the Chronicles (Cedoc), Faculty of Medical Sciences, Universidade Nova de Lisboa.

Therefore, the tests with the PCR technique that the hospital laboratories and the National Institute of Health Dr. Ricardo Jorge (Insa) have applied to the diagnosis of covid-19 in Portugal.

Does the PCR technique allow for quick results?
Yes. In addition to the conventional, more time consuming PCR technique, there is real-time PCR (RT-qPCR) and allows results to be obtained in three hours, or less. When the Prime Minister, António Costa, this week spoke of an order for rapid tests, it was this type of tests with the RT-qPCR methodology he was referring to. According to the press office of the Ministry of Health, RT-qPCR is currently being used in the diagnosis of the new coronavirus in five Portuguese hospitals (Centro Hospitalar do Porto; Hospital de São João; Centro Hospitalar de Vila Nova de Gaia / Espinho; Centro Hospitalar de Lisboa Central and the Portuguese Institute of Blood and Transplantation).

The 280,000 diagnostic tests ordered by Portugal, to combat the pandemic, will also look for the genetic material of the virus?
Yes. The technique involved in the tests ordered is PCR, so both the diagnostic tests currently available in the country and those that will arrive identify pieces of the virus's genetic material. “RT-qPCR captures the time window in which the person has a viral load detectable by the test”, explains Vasco M. Barreto.
Therefore, all these tests in stock and ordered do not detect the presence of antibodies in the patient's samples - molecules produced by the immune system in response to infection by the virus.

“The tests ordered by the Ministry of Health, through the Shared Services of the Ministry of Health, are all PCR tests, which in real time have high sensitivity and specificity”, says the press office of the Ministry of Health. distributed by different laboratories in the country, depending on their needs and compatibility with the equipment. ”

Why are antibody detection tests not already applied at this stage of the pandemic?
Antibodies produced by the immune system of an infected person in response to the virus do not appear until about ten days after infection. Therefore, antibody tests "only" say that the person has been in contact with the pathogen, in this case the new coronavirus. In other words, the person may be still infected - the virus still remains in his body -, as he may no longer have the infection and the memory of that infection will remain in his immune system through the presence of antibodies, giving him some degree of immunity.
In the last situation described, the person is no longer ill. In this phase of combating the pandemic, with social confinement in Portugal and in many countries, what matters is to know very quickly is whether the person is sick and if it represents a risk for himself and for others. At a later stage, antibody tests are important to assess, for example, the immunity acquired by each of us individually and as a group. These tests look for antibodies in people's serum, hence they are serological tests, and they are also quick.

What is the difference between antibody tests and antigen tests?
Lately, there have been reports of either antibody testing or antigen testing. Although they may look different, in reality it is the same test. In other words, they are two sides of the same coin.

The antigen is a particle on the surface of a pathogen, for example viral proteins. When they enter our body, these particles are recognized as foreign entities and the immune system attacks them. One form of attack is through the production of antibodies by one of the types of lymphocytes in our immune system, explains Vasco M. Barreto. Now this type of lymphocyte has the capacity to produce specific molecules - the antibodies - for that antigen. Made to measure, these antibodies fit perfectly into the antigen of that specific pathogen.

Once the infection has passed, this memory remains in the immune cells that produced specific antibodies to a particular pathogen.

To detect a pathogen - for example, the new coronavirus - the antigen that is recognized by the antibody is included in the test. “The antigen is like the bait we give to the sample [collected from the patient]”, explains Vasco M. Barreto. If the sample contains antibodies, interaction with the antigen is established. While the antibodies are in the patient's sample, the antigen is in the diagnostic test.

News from the newspaper Público
Teresa Firmino
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