Tuesday, November 24

Antigen tests are not very efficient in mass screening | Madrid

On September 30, the Executive of Isabel Díaz Ayuso began the use of antigen tests for mass screening in the general population, much cheaper (4.5 euros) than a PCR (around 18 euros) and much faster in giving results , about 20 minutes. Since then, 1.3 million citizens have been summoned in 29 municipalities, but less than a third have attended and of the 403,373 tests that have been done so far, 1,513 have been positive. On the contrary, where they were effective was in their use for symptomatic patients in primary care centers, hospital emergencies and residences.

Experts maintain that these tests are not indicated for the general population nor are they efficient for the control of the pandemic. “What are they for?”, Asks Fernando García, epidemiologist and spokesman for the Madrid Public Health Association, “for what they are indicated because it has been previously studied that they do work.” In environments with a foreseeable high prevalence, such as hospital emergencies, primary care and nursing homes; for people with symptoms in the first five days of disease progression, in addition to close contacts and vulnerable or exposed populations, such as health workers. “There, yes, and the Community did well by anticipating the purchase of these tests and starting them where they had to be, but not in the screening. These data show that they are of little use in the control of the pandemic ”, says García, who refers to the fact that of the 75,009 new cases diagnosed since September 30, the results of these tests account for 2%.

0.17% of positives in Torrejón or Morata de Tajuña, 0.30% in San Martín de Valdeiglesias or 0.40% in Madrid are, for García, one of the indicators of why the use of AG in the screened, “as it is raised, it does not make sense”: “When applied indiscriminately, there is a suspicion that the sensitivity drops a lot. It is something that has not yet been studied and it is another reason not to use them where it does not correspond ”. He explains that, as the frequency of infection decreases, the predictive value changes, both positive and negative, because the test is “imperfect”, it neither diagnoses all the positives nor discards all the negatives: “But above all the positive, and, even more, with a prevalence below 1%. In that case, the estimate is that the capacity of the tests falls below 50%. That is, if you have 100 positives, at least half could be false positives ”.

Unnecessary quarantines

This means that, if this new contagion is not confirmed, in this case with a PCR test, “you may be generating isolations and quarantines of people that do not have to be done.” To the there is no specific section on its application in asymptomatic or screened people in the general population In the protocol for the use of these tests, there is no established procedure and it is unknown how many of the positives that come out of these tests undergo a PCR to confirm. “It is done on a clinical basis,” says a spokeswoman for the Ministry of Health.

Javier Padilla, a primary care physician, recalls that the Community decided on September 27 not to test positive for close contacts, first in schools, and three days later, for the entire region. Since September 30, only those who present symptoms, live with the newly diagnosed, are members of vulnerable groups such as the elderly or chronically ill or essential workers are tested.

“What is the point of stopping tests in these areas and being screened in the general population?” Asks Padilla, pointing out that 400,000 tests in two months cannot be considered a massive screening. A volume that represents half of the tests and in eight times longer than what Isabel Díaz Ayuso announced, who on September 18 assured that in a week they would perform antigen tests on 857,193 people, 13% of the population of the Autonomous Community . It was an announcement that was not fulfilled, “but it also did not make sense that it had been raised”, reflects this specialist.

Manuel Franco, epidemiologist, professor at the University of Alcalá de Henares and adjunct at the Johns Hopkins School of Public Health, points out that the low citizen response “gives an idea of ​​the validity of these screenings, it is not useful for a study, since representativeness is not well designed. You cannot jump from municipality to municipality with a response percentage like that. You know what happens to a tiny percentage of the population ”. “Madrid is reducing its cumulative incidence, the health system is beginning to breathe, it is time to reinforce primary care and public health so that this does not happen again. And it is also the time to start recovering within the system everything that is not covid, which is what is going to do us more damage in the long term, “he concludes.

Information about the coronavirus

– Here you can follow the last hour on the evolution of the pandemic

– This is how the coronavirus curve evolves in Spain and in each autonomy

– Download the tracking application for Spain

– Search engine: The new normal by municipalities

– Guide to action against the disease

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