Rapid coronavirus tests at two speeds

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Catalonia has postponed the use of rapid antigen tests to detect the covid-19. But other communities such as Madrid or Andalusia have already used them for days, shortly after the Ministry of Health approved their indication. These new tests, whose main advantage is the speed of diagnosis (in about 15 minutes the result is obtained, compared to the between 24 and 48 hours that a PCR requires), show once again, territorial division that exists around the management of the pandemic: if some autonomies have already launched to use them in hospitals, health centers, schools and even in mass screening, others have decided to stop.

The Ministry of Health had the intention of implementing them this week only in hospitals and primary care centers (CAP), as well as in their respective emergency services, but finally their use will be delayed a few days or weeks yet to be determined, as the Secretary of Public Health of Catalonia recognized this Tuesday at a press conference, Josep Maria Argimon, and the director of the new Covid Monitoring Unit, Jacobo Mendioroz.

According to Argimon, Salut is awaiting a report from a commission from the Catalan Health Service (CatSalut) and that the computer system used for PCR with which it will be used for antigen tests has just been integrated. But Andalusia, for example, he is already using them in hospital emergencies, health centers and nursing homes. And not only that: but he has just changed the school protocols and, from now on, when a student presents symptoms, they will be transferred immediately to the reference health center to undergo a rapid test, Julia Camacho reports. If it is ruled out that it is a positive in covid-19, the minor will return to the center without interruption or isolation from their companions. For its part, Madrid has acquired five million rapid antigen tests and began to use them on Monday of last week, both in health centers and emergencies and in mass screening in neighborhoods with the most cases, reports Manel Vilaseró.

“It is true that antigen tests are already used in other communities, but we are not exactly the territory that does the least tests [en las últimas dos semanas, Catalunya realizó 30.000 PCR]. We are in a hurry to use them, but we have to try our best, “Argimon explained.

The Generalitat de Catalunya has bought one million antigen tests rapid diagnosis of covid-19 to the pharmaceutical Abbott, of which some have already arrived 600,000 units which are expected to be used soon. According to Salut, sensitivity of these rapid tests is between the 96% and 97%. Although the most reliable coronavirus tests will continue to be PCRs, one of the advantages of antigens is that they will help decongest the system because they will prevent the symptomatic patient from self-confining at home while waiting for the test result.

Like PCR, antigen tests consist of a nasopharyngeal sample. And they have another great advantage over PCR: the price: A rapid diagnostic antigen test costs Salut a few 4,5 euros; a PCR test, about 96.

Scientific endorsement

Catalonia has validated the rapid diagnostic antigen tests in the laboratories of the Vall d’Hebron, Bellvitge and Can Ruti hospitals (Badalona). In addition, the reliability of this new covid-19 detection tool is also endorsed by a report from the Carlos III Health Institute of Madrid. That is why Salut finally will not run a pilot test of these tests in primary care, as planned at the beginning, and will start using them directly only in people with symptoms compatible with covid-19: the group in which the reliability of these tests has been tested.

The sensitivity of antigen tests is 96%, as long as they are used between the third and fifth day from the onset of symptoms

Thus, unlike Madrid, Catalonia will only use rapid diagnostic antigen tests in hospitals Y CAP. Those areas in which massive screening is carried out (such as, for example, schools or neighborhoods where there are outbreaks), which are intended to detect asymptomatic positives and thus cut the transmission chains, will be excluded.

“Rapid diagnostic antigen tests they do not come to replace anything, but they must be a complement of PCR tests “, he assures Robert Fabregat, the general director of Research and Innovation of the Department of Health. “Perhaps we will have to validate positive cases with a PCR, but negative cases can be automatically discarded,” he adds.

Antigens and PCR: differences

Antigen tests and PCRs are direct tests that detect if there is a virus membrane protein (or a viral particle). Antibody tests would be an example of indirect tests, because they analyze immunity (antibodies) based on whether the disease is active or past. “The antigen looks at the protein of a membrane, but does not amplify it. PCR is very useful because, although there are very few viral particles, it amplifies them and is thus easier to visualize. That is why sometimes we have so many false positives,” he says Judit Villar, Doctor of Infectious Diseases at the Hospital del Mar in Barcelona.

Thus, since the antigen is capable of detecting a virus particle but is not capable of amplifying it, it is very important to perform this test in the “right moment”. “The sensitivity of the antigen depends on the exact moment, so it is important to establish a protocol on when it should be used. The antigen test should be done between the third and fifth day from the onset of symptoms, if not, the sensitivity drops a lot, “says Villar. If the antigen test is used well, it has a high sensitivity.

PCR, unlike antigen, amplifies viral particles – that’s why it leads to so many false positives

For this doctor the advantage of antigen tests is speed. “Further, no specialized personnel are needed, since the PCR analysis requires a microbiologist, while the antigen test only needs a few drops of a reagent liquid “, he says.

Antigens have disadvantages too. For example, in asymptomatic people “its reliability has not been proven”. For Villar, these tests are “very useful” in emergency services or health centers where “rapid tests” with “high sensitivity” are needed to decide “what to do with patients”. “If the clinical suspicion is high and the antigen is negative, we will have to do PCR. Antigen tests will not save us certain PCRs, but they will save us those in which the antigen is positive”, concludes Villar.



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