Beyond the positive: the information of the PCR that allows to know the contagion capacity | Radio Madrid



Positive or negative. Those are the main results that a PCR can give. However, these tests can also provide information beyond the mere confirmation of cases, which can be observed through the so-called cycles.

This indicator establishes the difficulty —or ease— of finding the virus in an infected person, so that the greater the number of cycles, the lower the viral load of a person; as stated by the virologist and professor of microbiology at the Autonomous University of Madrid, José Antonio López Guerrero. “It is assumed that after 25 or 30 cycles there is so little viral load that even, many times, it is not possible to know if the virus is being detected or simply noise, remains,” he says.

The expert’s statement is also included in the Strategy for the early detection, surveillance and control of COVID-19, of the Ministry of Health. In fact, about the social and health personnel the document establishes that they will be able to return to work – if they have not required hospital admission – “after 10 days from the onset of symptoms provided they remain without respiratory symptoms or fever for at least 3 days” and after performing a “negative PCR or that, even if it is positive, it is positive at a high cycle threshold (Ct) “.

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This high cycle threshold referred to by the Ministry is also detailed in the document. “According to the scientific evidence available, it is assumed that a cycle threshold (Ct) higher than between 30 and 35 would be equivalent to a viral load without infective capacity. However, this criterion must be validated by the responsible laboratory in each case”, reads the ministerial document.

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The strategy contemplated by the Ministry of Health is also applied in the Community of Madrid with regard to social health workers. In fact, the Ministry of Health assures that the protocol is “the same” as that of the Ministry, and, therefore, “if a control PCR shows high cycles, higher than 30, it is considered non-contagious” to the worker.

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Along the same lines, as the newspaper advanced The country, the Madrid City Council allows Firefighters, Municipal Police and Samur workers to return to their jobs provided that the PCR reflect a high number of cycles. However, they point out from the Consistory there is no employee who works being contagious. “The protocol is the same as that established by the Ministry of Health and no one works below 30 cycles. It is the protocol in force and is applied by doctors, not the managers of the City Council,” they assure.

The virologist López Guerrero points out that a person with symptoms who is in these cases, that is, who has tested positive and has passed his quarantine, the detection tests that offer high cycles are quite reliable. “A detection at very high cycles of PCR will mean that practically only traces remain. Zero risk will never exist, but it can be considered that it is a safe person; although it is necessary to try to maintain control and hygiene measures,” he emphasizes.

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However, the expert points out the case of asymptomatic patients as something more complex. “A person who has not presented symptoms should be kept isolated to repeat the test after a few days since, if the cycles go to lower levels between two CRP, it would mean that that person was initiating the infection. However, if the cycles continue to rise level, it means that he is finishing it, “he assures.




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George Holan

George Holan is chief editor at Plainsmen Post and has articles published in many notable publications in the last decade.

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