PCR, antigen tests, serological tests … How are they, what are they for (and what not for) each of the tests that detect the coronavirus

Even Rosalía had her test to detect the coronavirus wrong this summer. And is that between some options and others it is difficult to clarify which type of test is better. Those who have already gone through the annoying PCR sigh for the quick and comfortable saliva tests at London airport and while, on Twitter, the networks do not stop pouring unfavorable comments if the new antigen tests that have been approved are of much use, Little or nothing. It is the best time to review everyone the tests that we have to detect the coronavirus and that it is clear to us once and for all what they are for, how they are done and in which cases one or the other is used.

What does each type of coronavirus test detect?

To understand the differences between the different types of tests we must take into account what the coronavirus is like. If you have seen a newscast in the last six months you will have become familiar with its shape: a small ball covered with "spikes". As well, Inside that ball is the DNA of the virus and those "spikes" that surround the outer layer of the virus are the famous antigens.

Antigens are the "keys" that the virus uses to enter our cells. And why do you want to enter them? Well, once inside the virus uses the resources of the cell to replicate their own DNA and thus multiply and expand throughout the body. Antigens are very useful for the virus, but they are also detectable by our defenses.

When the defenses locate antigens, they know that something is wrong and launch an immune response: this is when the antibodies, whose function is to "cling" to the virus antigens, come into action so that they are "marked" so that the rest of the body's defenses can destroy that virus.

As well each type of test measures a specific thing in this entire processThat is why a PCR is not the same as an antibody test, nor do they offer the same information.


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What do the serological tests against the coronavirus detect?

Serological tests locate in a blood sample the amount and type of antibodies that are circulating through our body. Depending on the type of antibodies that are detected in this test, we can know at what moment of the disease we are or if we have already passed it.

It is not useful for the most initial phases of the infection, when the virus is in our body, we can spread it to others, but our defenses have not yet detected it, but it is very useful, for example, in general screening to know data such as how much population has overcome the disease without symptoms.

What does CRP measure and when to use it?

PCR us indicates if we have the virus in the body (and therefore we can infect others) because what it detects is the genetic material of the virus. The PCR test is still considered, today, as the most reliable test to know if the virus is still with us, something especially important in asymptomatic people, who have a small amount of the virus in their body.

The cons of this test is that it has to be done by medical personnel because the sample being analyzed is not easy to obtain (and a bit annoying): a quantity of oropharyngeal mucus from the deepest area of ​​the nostrils needs to be taken with a swab, which is not exactly pleasant. In addition, each PCR test costs around € 100.

What does the new rapid antigen test measure and what is it for?

It also helps to know if we have the virus in the body, but in this case obtains the information by measuring whether there are coronavirus antigens in the blood. The new rapid antigen tests are similar to PCR in terms of how to extract the sample to be analyzed (with a swab through the nose), but They have in their favor that in just 15 minutes you have the result. These types of tests have been the last to be incorporated into the battery of tests to detect the coronavirus, but they have come with some controversy.


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Its favorable points include that they offer a much faster result than a PCR (which can take days), that they are cheap and that they have been shown to work very well in people who have developed symptoms. Do you know that question about how to distinguish if what happens to you is a cold or a coronavirus infection? Well, in people with those symptoms of the beginning of the infection rapid antigen tests have proven to be very useful in determining whether the culprit of the malaise is the coronavirus or not.

The problem comes when it comes to screening asymptomatic people who have a low viral load: There is not yet enough data to know if rapid antigen tests are as effective as PCR in these casesIn fact, the WHO continues to consider the latter as the most reliable.

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