Coronavirus Test The diagnosis of those affected by COVID-19 is crucial to adequately treat the disease and stop the transmission of the new coronavirus. Since the beginning of the epidemic, diagnostic tests have been used, but the type of tools used and the criteria for selecting the people who should undergo these tests have varied. Now that we are in the worst moments of the pandemic, we explain to whom they are made and what devices are used.
Health professionals are in charge of deciding, with clinical criteria, who undergoes this type of tests, following the guidelines set by the authorities. According to the Ministry of Health, in a pandemic situation, conducting coronavirus tests should focus on two priority groups:
People with acute respiratory infection with criteria for hospital admission. In this way, transmission in hospitals is avoided. People with acute respiratory infection who are part of health personnel, socio-health personnel or other essential services. This guarantees essential services and protects vulnerable groups, controlling transmission.
However, these priority criteria have been expanded for various reasons, including that the World Health Organization (WHO) has recommended mass testing to all countries and the Government has acquired a considerable number of so-called rapid tests .
For this reason, diagnostic tests are also considered for particularly vulnerable people (such as the elderly in nursing homes), even if they are not serious. In short, people with mild symptoms are no longer ruled out. But always after an individualized clinical evaluation, two types of tests are used to diagnose SARS-CoV-2 infection, which is the coronavirus that causes the disease called COVID-19: PCR and rapid tests.
PCR, acronym in English for polymerase chain reaction, is a technique that allows to detect and amplify a fragment of the genetic material of viruses, bacteria or other pathogens. In the case of the coronavirus, what is located is an RNA molecule.
To perform it, a respiratory sample of the person suspected of being infected is required. For example, a nasopharyngeal exudate can be obtained, that is, a sample of the secretions from the upper part of the throat. If the test is positive, that is, when RNA from the virus is detected, that individual is confirmed to be infected with SARS-CoV-2.
It is a method of common and habitual use in the microbiology laboratories of hospitals and research centers. However, it presents a certain degree of complexity, so it can only be carried out by trained and prepared personnel.
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Among the fundamental advantages of PCR, its high specificity stands out, since it can differentiate between two evolutionarily very close microorganisms; high sensitivity, as it detects amounts of 20 copies / ml, or even less, of viral genetic material; and its early character because it detects viruses in the early stages of respiratory infection.
PCR has a disadvantage in an emergency and pandemic: it takes several hours to determine if a person is infected or not, which, in turn, limits the number of diagnostic tests that each laboratory can perform per day. On the contrary, the so-called rapid diagnostic tests allow knowing in 10-15 minutes whether a person is infected with the new coronavirus. So why hasn’t this technique been used so far? The Ministry of Health responds in a statement that “until a good number of infected patients were available to characterize the antibodies, it could not be developed.”
PCR is a direct method of diagnosis, while rapid tests are indirect. This means that they do not identify the RNA of the virus, but they detect either the antibodies produced against the virus using a blood sample, or virus proteins present in the respiratory samples of nasopharyngeal exudate.
These tests have another advantage over PCR: they can be performed at the home of a suspected case, always supervised by a healthcare professional. They are also being implemented outside some hospitals in certain autonomous communities, without the need for those who submit to them to get off their vehicles. Rapid tests are based on what is known as paper immunochromatography, that is, a platform that has attached the virus proteins to detect antibodies, or specific antibodies to detect virus proteins.
But, since its sensitivity and specificity are not as high as those of PCR, when it is performed on a large number of people, it should be considered, in a way, as a screening method, which will allow limiting PCR assays to those patients. with symptoms that give a negative result in the rapid tests. It is hoped that this will free up professionals and resources from the health system to be able to dedicate them to other equally pressing tasks.