Fact Check-The WHO did not say PCR tests are ‘invalid’; a Canadian article has misrepresented quotes in a WHO notice written for professional scientists 

An article shared on social media has falsely claimed the World Health Organisation (WHO) has admitted numbers of COVID-19 cases are “meaningless” due to “invalid” PCR tests. The WHO told Reuters this claim is not true.

The article was originally posted to (here), a website that describes itself as a Montreal-based research and media organisation that publishes the “unspoken truth” ( Its headline, a screenshot of which has been shared repeatedly on Facebook (here , here , here , here , here , here), reads: “The WHO Confirms that the Covid-19 PCR Test is Flawed: Estimates of ‘Positive Cases’ are Meaningless. The Lockdown Has No Scientific Basis.”

“The World Health Organization (WHO) tacitly admits one year later that ALL PCR tests conducted at a 35 cycle amplification threshold (Ct) or higher are INVALID. But that is what they recommended in January 2020, in consultation with the virology team at Charité Hospital in Berlin,” the wider article says.

“If the test is conducted at a 35 Ct threshold or above (which was recommended by the WHO), segments of the SARS-CoV-2 virus cannot be detected, which means that ALL the so-called confirmed ‘positive cases’ tabulated in the course of the last 14 months are invalid.”

To back up this claim, the writer then points to what he says is a “retraction” released by the WHO in January (here).

However, this is not true. The WHO information notice is not a retraction; rather, it is a clarification for laboratory professionals on how to interpret PCR results. It does not mention anything about “35 cycle amplification threshold”, nor does it say PCR test results are invalid.

In an email to Reuters in February, the WHO said: “Since the beginning of 2020, WHO has received 10 reports of problems related to PCR tests for the detection of SARS-CoV-2 (the virus which causes COVID-19), including some products listed for emergency use by WHO. The reports were for misdiagnosis, both false positive and false negative results.

“After thorough investigation, WHO confirmed that tests were not always being used appropriately and in accordance with the instructions provided by the manufacturer. In particular, users in laboratories encountered problems with these tests when they did not apply the recommended positivity threshold – this can result in either false negative results (if the threshold applied is lower) or false positives (if threshold is higher).”

PCR stands for Polymerase Chain Reaction, a process that is repeated multiple times to amplify genetic material of the virus in a sample. Scientists often run between 40 and 50 cycles of the process, depending on the laboratory and testing kit (here, here).

Ian M. Mackay, a virologist and adjunct associate professor at the University of Queensland in Australia, told Reuters in February that reducing the cycle count (as suggested in the article) would change almost nothing. In other words, a high majority of results would still return as positive and be included in the case count.

“This is because most PCR results don’t fall at or above 40 cycles – the usual endpoint of a real-time PCR,” Mackay said. “They fall at a much earlier cycle number.” This is explained in more detail here and in another Reuters debunk on the topic here .

The threshold cycle refers to a specific point in the test where the positive result occurs, explained Mackay. At that point, the fluorescence signal from the PCR test crosses a certain threshold, which can be determined by the user or the manufacturer of the test. The value can either be fixed or adjustable by the user. “The WHO are saying that if the manufacturer has defined a value but you could if you chose to adjust that setting, please don’t - stick to what the manufacturer has stated because they have done the earlier work to determine the best value for that threshold,” said Mackay.

The WHO added: “We saw a small number of instances where users were not following the instructions to the letter. Millions of PCR tests have been conducted properly with accurate results.”


False. The World Health Organisation did not issue a retraction about PCR tests to say all positive results were invalid; rather, the organisation released a clarification for professional scientists advising them to follow instructions from test manufacturers.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .