re: PCR test gives false positives from colds and other viruses now confirmed.

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However, Dr Thushan de Silva, from the University of Sheffield’s Department of Infection, Immunity and Cardiovascular Disease, has previously told Reuters that this is not correct.

He explained that the document is outlining a “very common process” that is used to determine the lowest amount of viral genetic material the PCR tests could detect. To calculate this limit, he said:“You need to have a known quantity of virus to extract genetic material (RNA) from, or alternatively a known quantity of RNA identical to that carried by the virus”. In this case, the identical RNA that the CDC used was ‘transcribed’ RNA. This is synthetically produced genetic material which is identical to that carried by the virus, and, according to Dr Stephen Griffin, a virologist and Associate Professor at Leeds Institute of Medical Research, means that scientists already know how much genetic material is present in the sample.

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He explained “you can't make it quantitative with a virus stock that's not been characterised. So you start with an in vitro [in the lab] transcribed RNA, it’s much better, it's easier to do that."

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Dr De Silva told Reuters: “There are now hundreds of stocks of cultured SARS-CoV-2 in laboratories around the world”.

Dr Griffin previously told Full Fact: “SARS-CoV2 has been sampled millions of times over from infected people, including those originally found to be infected in China,”.

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