Antigen tests, much faster and cheaper than PCR, are increasingly reliable and their extensive use may represent a turning point in the control of the Covid-19 pandemic.
Two new investigations, carried out by Spanish scientists and which have yet to be peer reviewed before being published in scientific journals, have shown that a specific model of this type of test – that of Panbio – produces an insignificant amount of false positives while false negatives are restricted to patients who do not spread the disease.
“PCR is so sensitive that it sometimes detects people who are not infectious. The antigen has the advantage that it does not detect them”, says Oriol Mitjà, doctor at the Germans Trias i Pujol Hospital and author of one of the two studies , whose preprint was published this Wednesday in medRxiv.
“The other outstanding point is that we use a nasal brush, a non-invasive method that allows self-sampling. The same person puts the stick up his nose and does not have to put it inside,” explains Mitjà.
A second generation
Antigen tests were widely rejected by the scientific community at the beginning of the pandemic due to their low reliability, but a more developed second generation has been in force since September in the epidemiological control strategies of many countries.
The WHO does not recommend using this type of test in asymptomatic patients, except if they are confirmed positive contacts.
The Carlos III Health Institute issued, at the end of September, a validation report on the antigen tests manufactured by the pharmaceutical companies Abbot and Roche, which compared them in sensitivity to a PCR test in the case of symptomatic patients.
The clinical laboratories, however, then alerted non-symptomatic patients to the risk of false negatives that could generate “false security”.
According to Mitjà, his study certifies that those known as false negatives by antigen tests that, however, would give positive in a PCR, are due to a “low viral load” – which would not generate infections – and “have nothing to do with the patient is symptomatic or asymptomatic “.
The other study on this type of test, prepared by scientists from the Hospital Clínico Universitario de Valencia and also published as preprint last Tuesday in medRxiv, reaches a similar conclusion: “Our data suggest that patients who have tested positive in PCR and negative in the rapid antigen test are unlikely to be contagious.”
Madrid’s successful strategy
If the hypotheses of these two studies are corroborated, the consequences for the strategies to control the pandemic could be enormous. The main advantage of this type of test compared to PCR is its low cost and, above all, its speed.
While a PCR test, with the current overload of laboratories, usually takes more than 24 hours to give results, a rapid antigen test provides them in 15-20 minutes. This difference is key when it comes to stopping the chains of contagion early.
The region that leads this type of test in Spain is the Community of Madrid, where the pandemic is being controlled much more effectively than in the rest of the country for weeks.
“I think that the reason why Madrid has not had the escalation of cases that we have seen in many other parts is because at that time Madrid is already doing antigen tests in a massive way and in a preferential way to a PCR, perhaps without knowing what we know today or simply suspecting it ”, says Saúl Ares, a scientist at the CSIC.
Creation of “safe spaces”
Beyond its current application, for Mitjà, its extensive use could avoid new confinements while waiting for the vaccine to be ready by creating “safe spaces” that allow the economy and social life to return to a certain normality.
“This is a bomb if you buy enough quantity. The utopia is that there would be enough tests for the entire population and no further confinement would be necessary,” says Mitjà. “But a more realistic strategy is that all people who are going to do a risk activity are tested, any activity where people get together: go to eat at a restaurant, play a game, go to the gym or go to a concert. “