Coronavirus | Antigen test in Madrid: how the test is done, price, reliability and differences with PCR

In mid-September, Health approved in Spain the incorporation of the antigenic test as a diagnostic and screening tool after it was requested by many autonomous communities. Although experts have begun to warn about their use, they have several advantages and can be very useful in certain situations. Madrid has announced that it intends to establish them systematically to carry out a massive screening of the entire population, with the intention that it be before Christmas. What does it consist on?

Characteristics of antigen tests

The main difference antigen tests have with respect to PCR is the speed at which they can be obtained. results, since antigen tests take between 12 and 15 minutes to provide them, while with PCR you have to wait about 4 hours to know the result, although due to laboratory delays, we will know if we are positive or negative within one or two days.

On the other hand, the antigen test is easier to do Y does not require the presence of health personnel. To do it, you have to take a sample from the mucosa of the nose using a hyssop (a thin stick with a small cotton ball at the end), and place it in a piece of cardboard the size of a credit card that contains a reagent. After 12-15 minutes, the results can be read directly on the cardboard. A line color will indicate that the result is negative, while if they leave two lines means that we have given positive.

These rapid tests are also notable for their low price, that usually haunts 4.5 euros, compared to the almost 100 euros that it costs to carry out a PCR.


The efficacy of antigen tests can be greater than 95%. However, from the National Business Association of Clinical Laboratory (ANLAC) they report that their reliability is so high in cases of high viral load, if performed between three to ten days after infection due to coronavirus, which is when the person is most contagious, or when the patient has symptoms. These tests can be very helpful in these situations.

Instead, they are “Unreliable in detecting cases found in the first days of infection (just before the period of greatest contagion capacity), when they have elapsed more than five days after the onset of symptoms and, above all, in those with a asymptomatic course of the disease ”, they specify from the same association. In these cases, therefore, it would be more convenient to perform a PCR.

For its part, the Spanish Society of Public Health and Health Administration (Sespas) advises that these antigen tests could give more false negatives than PCR tests, so they insist that they should not be used as a replacement for PCR and that they will only work well in certain settings.

“They are not suitable for all situations, they lose sensitivity when these tests are taken in asymptomatic people and also when the probability of infection is very low. Therefore, these tests have a priority use in symptomatic people, but are not good at screening in the general population. They have to be interpreted very carefully ”, recently commented Fernando Simón, the director of the Center for the Coordination of Health Alerts and Emergencies of the Ministry of Health.