Antigen tests in Spain: Sales rocket but experts warn they create false sense of security

The ongoing sixth wave of the coronavirus in Spain has spurred the sale of home-testing antigen kits. In November, when the epidemiological curve began to rise once more, 1.3 million tests were sold by the country’s pharmacies – that’s 115% more than the previous month, according to data compiled by health consultancy Iqvia. And this trend has continued in recent weeks: the approaching holiday season and the rise of this sixth wave – the 14-day incidence per 100,000 inhabitants is currently 442 – is still encouraging the purchase of these kits, which, while they can be used to detect positive cases, are not infallible. Epidemiologists warn that they can give false negatives and also can create a false sense of security. Such tests do not exempt people from observing other preventive measures, such as using masks indoors or in crowds, maintaining social distancing and ventilating enclosed spaces.

Antigen tests have been available in Spain with no need for a prescription since this summer. These tests identify the presence of the coronavirus’s proteins, and according to the Spanish Medicines and Health Products Agency (AEMPS), there are around 20 different types available in the market.

Consultancy Iqvia reports that in 2021 a total of 8.7 million tests were sold in pharmacies, but that sales rise or fall according to the epidemiological curve. As such, during the fifth wave in July and August, 1.5 and 1.9 million tests were sold, respectively, while in September and October, when the curve fell and the virus was not circulating so freely, sales fell to fewer than a million each month. Now, with the rise of the sixth wave, Christmas approaching and the threat of the omicron variant, which the experts fear is more transmissible, sales are up once more.

Two weeks ago, the Madrid premier, Isabel Díaz Ayuso, announced that every resident of the region would be able to get one antigen test each for free, “so that social gatherings can be held safely,” she explained at the time. But epidemiologists consulted by EL PAÍS distrust the mass use of these diagnostic kits, because, they warn, they can create “a false sense of security.”

We need to stop selling these self-diagnostic tests as a silver bullet because they do not give a definitive and robust response

Daniel López-Acuña, former director of emergencies at the World Health Organization

Daniel López-Acuña, a former director of emergencies at the World Health Organization (WHO), points out that the kits “do not have the same specificity and sensitivity as a PCR test.” He is referring in the former case to the ability of the product to recognize the absence of the marker linked to the disease, and in the latter to the ability of the test to detect the presence of that marker. He is calling for caution. “We cannot promote the idea that they offer a definite yes or no. And they cannot be an excuse not to get vaccinated, not to have the Covid certificate in force or avoid the use of masks, distancing and ventilation among groups of people who do not live together.”

Antigen tests are not infallible, is the message. They can give false positive and negative results. The board of experts advising the Spanish Health Ministry already pointed out this summer that a positive test via these kits would be officially considered as a suspected case, and that the infection would have to be confirmed via another means. There is currently no system in Spain to officially count results from home antigen tests – it is down to individual responsibility.

According to the experts, pharmacies should give clear information to customers on how to proceed should the test come back positive. But the final decision on what to do is taken by the user. Some regions, according to the General Pharmaceutical Council, have rolled out a protocol to facilitate the possibility of registering the result of the tests from pharmacies, assuming that the purchaser wants to do so. The government’s experts have, meanwhile, stated that if a person tests positive via an antigen test they should begin isolating at home and get in touch with their regional healthcare authorities in order to confirm their coronavirus infection according to the protocols in place.

The document produced by the government’s consultants also warns that “negative results do not exclude the possibility of an infection.” This is what happened, in effect, in the case of the mass outbreak among healthcare staff in Málaga, where all of those who attended an organized dinner had taken antigen tests beforehand, but in some cases, there were false negatives – more than 80 people ended up infected. This also was the case at three music festivals that were held last June in Catalonia: antigen tests were carried out before people could enter, but more than 2,000 people ended up infected, according to a study by the regional government.

So what do antigen tests serve for? They are another level of protection, such as masks or social distancing, according to Óscar Zurriaga, the vice-president of the Spanish Epidemiological Society. “The message has to be that all measures are important: take a test, but keep all the others in place. Basing everything on a single measure is a bad idea,” the epidemiologist states.

If you have symptoms and test negative, it’s preferable that you confirm this with a PCR because this test only detects high viral loads

Salvador Peiró, epidemiologist from the Fisabio research foundation

In practice, these kits work best for symptomatic people, during the first five or six days after infection. But the predictive value of the tests “depends a lot on the epidemiological situation and symptoms,” adds Salvador Peiró, an epidemiologist from the Fisabio research foundation in Valencia. “If you have symptoms and test positive, it’s almost certain that you are positive,” he explains. “If you have symptoms and test negative, it’s preferable that you confirm this with a PCR because this test only detects high viral loads and if you get it at the start of the infection, during the incubation period, it might not detect the viral load.” But the situation is “much more complicated” with people who are asymptomatic, because there can be false positives and negatives. Peiró adds, however, that the more the virus is circulating, the lower the probability of a false result.

“It’s a mistake to use this measure to substitute others,” the Fisabio epidemiologist continues. “A test rules out some risks but doesn’t avoid all of them.” He warns that the incorrect use of these tests allows for the virus to circulate. “For example, if you have had close contact [with a coronavirus case], the antigen test will come back negative and you won’t isolate. If you had close contact yesterday, it will take four days before you come back positive. In the four or five first days of incubation, there is insufficient viral load for the tests to come back positive.” What’s more, there can be problems when carrying out the tests: their effectiveness has been tested in laboratory conditions, but people at home can do them wrong or contaminate the sample, thus altering the result.

López-Acuña insists that “we need to stop selling these self-diagnostic tests as a silver bullet because they do not give a definitive and robust response.” He also points out that, along with the high incidence in Spain, there is still the “uncertainty about the omicron variant.” This strain is suspected to be more infectious than the currently dominant variant, delta, and as such has the world on alert.