Covid-19: These are the differences between PCR, rapid test and serology

Now that the Covid-19 tests have become part of our lives, many questions arise: How do rapid tests work? What information does a serology give us? And the PCR?

The General Council of Nursing and its Collegiate Organization has released a video in which it intends to solve all the doubts of citizens about the different diagnostic techniques that are currently being used in the coronavirus and how to interpret the results.

PCR and new rapid test

There are diagnostic techniques, virological and serological, which are currently used to detect Covid-19. Among the first, the PCR stands out, which is currently the one that offers the greatest diagnostic certainty to find out if a person has the active infection at that time.

Virological tests are based on the analysis of respiratory tract samples. To take them, a swab is used – a kind of stick – and they are usually collected from the nose or mouth and throat, although in certain patients, for example, those more serious, the lower respiratory tract is usually used.

It takes several hours or even days to know the result of the PCR. To shorten times, work is being done on the introduction of new rapid antigen tests, which start from the same type of samples but are capable of providing a result in less than 30 minutes.

Serological tests

Serological tests are based on a blood sample, well venous – obtained by extracting vein blood– well capillary – through puncture on the fingertip-. These allow to identify if the person has been in contact with the virus and has generated antibodies.

Interpretation of results

The positive result obtained by virological tests is considered definitive. The negative, however, in some cases and after evaluation by the health professional, requires a repetition of the test.

In the case of serological tests, considering the positive or negative of the IgG and IgM, in people who do not have symptoms, it will act as follows:

-If the IgG is negative and IgM is positive It is necessary to confirm if the infection is active, therefore, health professionals will value the performance of virological tests.

-When the IgG is negative and IgM is negative it means that there has been no contact with the virus or no antibodies have been generated, therefore, in general, no further tests are required.

-Before a result of Positive IgG and negative IgM no further tests are required, as it means that the infection has passed.

-When both, IgG and IgM are positive, the health professional will assess in each case the need or not to carry out more additional tests.

When to do each test

“One of the main doubts that citizens have refers to why PCR is used in some patients and rapid tests in others. This is one of the aspects that we also explain when to resort to one technique or another, ”says Florentino Pérez Raya, president of the General Council of Nursing and its Collegiate Organization.

Thus, while the tests virological are performed when the person presents symptoms or have had close contact with someone infected, serological are used when you want to know if the person has been in contact with the virus and has generated antibodies (or for seroprevalence studies). Therefore, during the incubation period, approximately, in the first week after the possible infection, virological tests are used. After that time, serological tests are recommended.

As Florentino Pérez Raya explains, “our intention is for citizens to have simple and scientifically proven information about diagnostic tests because we are aware that at this time it is something that generates a lot of concern and constant doubts. To do this, our expert nurses have worked on adapting the technical content to a language easier for the population to understand. It is not a question of being experts in diagnostic tests, but of answering those doubts that, we know, are on everyone’s lips right now.

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