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Virologist Raquel Guiomar, from the Ricardo Jorge Institute, said on Tuesday that false positives in PCR tests for covid-19 screening may be due to a “small contamination” of the sample under analysis or to incorrect interpretation of the data.
Without commenting directly on the case of the President of the Republic, Marcelo Rebelo de Sousa, who returned this Tuesday to test negative for covid-19 after having tested positive after a first negative test, Raquel Guiomar referred that specific cases like the one that happened, where there are results that raise doubts, a “second evaluation” is required by the Ricardo Jorge Institute, a reference laboratory in Portugal.
The head of state tested negative for COVID-19 first with an antigen test and then positive with a PCR test. It came back negative with two new PCR tests. The first PCR test was carried out in a laboratory of the national network, the last two in the Ricardo Jorge Institute.
Antigen tests, which detect covid-19 coronavirus proteins, allow for faster results, being recommended, for example, to control outbreaks.
However, these tests are less sensitive, especially when the viral load is lower (which can occur at the beginning or end of the infection) and there are no symptoms, so they are less reliable than PCR tests, which detect small amounts of genetic material of the virus, being recommended, for example, to hospitalized or asymptomatic patients with high-risk contacts with confirmed cases of infection.
Raquel Guiomar, head of the National Reference Laboratory for Influenza Virus and other Respiratory Viruses of the Doutor Ricardo Jorge National Institute of Health, in Lisbon, told Lusa that, faced with an antigen test with a negative result for covid-19, the Guidelines in Portugal require that a new test be performed to confirm the results, in this case a PCR test (or molecular test).
Since the PCR test has given a different result, it must be repeated so that there is no doubt.
As possible justifications for a PCR test to have been (false) positive after a negative antigen test, the virologist points to “minor contamination” of the sample during laboratory analysis or “assessment of any signal” in the interpretation of the data.
Without citing names, Raquel Guiomar said that the Ricardo Jorge Institute “is trying to understand what happened” in the chain’s laboratory that did the PCR test that gave a false positive for covid-19 this Monday for the President of the Republic .
After two new PCR tests, one still last night and the other this Tuesday, the Ricardo Jorge Institute confirmed negative results for Marcelo Rebelo de Sousa’s covid-19.
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