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What are antigen tests? Are they as reliable as a PCR?

The Ministry of Health approved on September 22 the use of antigen tests to detect coronavirus. Now the Community of Madrid and Catalonia require that they be allowed to carry out this type of tests in pharmacies, although for now Health does not contemplate it. But what are these tests? And, above all, what is its reliability?

Antigen test: how it works

Antigen tests do not detect antibodies, but virus proteins. They are very simple to use, cheap, fast and, above all, the least invasive tests. Its price is around 4.5 euros, compared to the 100 euros that PCR can cost.

The procedure is very similar to that of the PCR test. A sample of the mucosa is taken with a cane. Then, instead of being sent to the laboratory for analysis, the cotton wool with the sample is placed on a piece of cardboard that is the size of a card.

In this piece of cardboard there are reagents, which indicate whether the result is positive or negative. It works like a pregnancy test: one line if the person does not have Covid-19, and two lines if they have the disease. The average waiting time to know the results is about 15 minutes, much less than PCR tests, which can take several days.

Positive antigen test results are very accurate. However, there is some probability of false negatives, so a negative result does not necessarily rule out infection.

Are they effective?

Many experts have doubted the effectiveness of PCR tests, although the truth is that over time they have been refined and improved. Now, as explained by Fernando Simón at a press conference, its effectiveness is not entirely proven in asymptomatic people because it loses sensitivity. Therefore, to this day PCR are still the most reliable tests, although it is true that antigen tests are very useful to decongest laboratories.

What are rapid tests?

The so-called rapid tests, which are being talked about a lot these days, allow us to know in just 15 minutes if a person has Covid-19 or not. Unlike the PCR tests, do not identify the RNA of the virus. If a blood sample is taken, they detect antibodies produced by the body against the coronavirus. Meanwhile, if a sample of the mucosa (antigens) is taken, the virus proteins can be detected.

These tests work like a pregnancy test. They make it possible to improve screening in the population, and limit PCR tests only to patients with symptoms of the disease.