PCR and rapid tests for the diagnosis of coronavirus, how do they work and how do they differ?

The researcher from the Virology Area of ​​the National Microbiology Center of the Carlos III Health Institute (ISCIII)Inmaculada Casas, who is part of the Coronavirus Technical Scientific Committee, explains the differences between the two techniques, and points out the importance of having more agile tools to promote the diagnosis of the disease.

The PCR, whose use is common and routine in the Microbiology laboratories of hospitals, research centers and universities, is based on the heat stability characteristics of a polymerase enzyme, whose discovery and subsequent application deserved the Nobel Prize in Medicine, awarded Kari Mullis and Michael Smith in 1993.

Through this technique, a fragment of genetic material is located and amplified, which in the case of the coronavirus is an RNA molecule. Casas points out that if, after analysis in a Microbiology laboratory of a respiratory sample from a person suspected of being infected, the test detects RNA from the virus, the result is positive and it is confirmed that it is infected with SARS-CoV-2. If the PCR technique does not detect the genetic material of the virus, the person would not be infected; When there is significant clinical suspicion, another test should be performed to ensure that the patient is not infected with the virus.

The PCR has a certain degree of complexity, so it needs trained and prepared personnel to carry it out. It has some basic characteristics that are: high specificity, since it can differentiate between two evolutionary close microorganisms; high sensitivity, since it can detect amounts of 20 copies / ml, or even less, of viral genetic material, and finally it is early because viruses are detected in the early stages of respiratory infection.

Since the beginning of the epidemic, diagnosis has been made using PCR techniques. Now they are beginning to carry out tests using a second battery of techniques, the aforementioned rapid diagnostic tests, which allow knowing in 10-15 minutes – the PCR takes several hours – whether or not a person is infected.

Why has this technique not been used before?

Until a good number of infected patients were available to characterize the antibodies, it was not possible to develop; PCR, being a direct diagnostic test, has been possible from the first moment the virus was sequenced.

The CNM expert points out that, unlike PCR, these rapid tests do not identify the RNA of the virus, but rather detect, or antibodies produced against the virus using a blood sample, which is another way of knowing if the patient is or has been infected, or virus proteins present in respiratory samples of nasopharyngeal exudate.

In addition to speed, these tests present another very important advantage at the present time since they can be performed at the home of a suspected case, always supervised by a health professional. They are based on a paper immunochromatography, that is, a platform that has the virus proteins 'stuck' to detect antibodies or specific antibodies to detect the virus proteins. Its operation is similar to that of pregnancy tests.

Thanks to these rapid tools, it will be possible to improve screening in the population and limit PCR assays only to those patients who, with symptoms, give a negative result through the rapid tests, which will allow the release of professionals and resources in the National Health System. .

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