Social media users have been sharing screenshots purporting that the World Health Organization (WHO) released guidance to laboratories and incorrectly saying this was to reduce the positive test result count in PCR (polymerase chain reaction) COVID-19 tests. The social media users claim the WHO’s guidance amounted to an admission that the current PCR tests were inflating COVID-19 infection number. This claim is false. The WHO’s guidance was meant to remind labs to conduct the tests with the instructions provided in order to ensure accuracy in the results.
Most posts include a misleading tweet by Dr David Samadi that reads: “The World Health Organization has now released guidance to laboratories around the world to reduce the cycle count in PCR tests to get a more accurate representation of COVID cases. The current cycle was much too high and resulting in any particle being declared a positive case.” The tweet no longer exists on Samadi’s Twitter page (twitter.com/drdavidsamadi).
The WHO notice does not advise laboratories to reduce the cycle count in PCR tests. It advises laboratory professionals to use tests with the proper instructions to ensure accurate results.
When contacted by email, the WHO told Reuters that it did not say the PCR tests for COVID-19 were faulty and that their guidance had been taken out of context. “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 organization said. “The reports were for misdiagnosis, both false positive and false negative results.”
The WHO confirmed after investigating that the tests were not being used in compliance with instructions provided by the manufacturers. Laboratories faced problems when they did not apply the recommended “positivity threshold”, which can result in false negative or false positive results.
Ian M. Mackay, a virologist and adjunct associate professor at the University of Queensland in Australia, told Reuters via email that the WHO notice is meant for laboratory professionals, especially those unfamiliar with conducting PCR tests in a pathology laboratory setting, and not the general public.
“If a lab uses a commercial RT-PCR kit, then they must follow the manufacturer’s instructions,” Mackay said, “Otherwise, the results produced may not be the best, most accurate results that the kit can deliver.”
In response to the claim in Samadi’s tweet that “the current cycle was much too high and resulting in any particle being declared a positive case”, Mackay said: “Apart from being completely wrong, this comment acts as excellent indicator of a person with no understanding of PCR, the design of PCR tests or of the high-throughout use of PCR processes to test human samples in a quality pathology laboratory setting.” He called the comment “bizarre” and scorned at how it was being used by adversaries of the PCR tests.
PCR stands for Polymerase Chain Reaction, a process used to amplify DNA, and it is run a certain number of times to detect a virus. Mackay discusses the process in detail here , explaining that to detect an otherwise small amount of viral DNA, laboratory professionals run 40 to 50 cycles of PCR, which can vary by laboratory and testing kit.
Mackay explained that reducing the cycle count would change almost nothing, a high majority of results will remain positive and those tested for the virus would still be counted as cases.
“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 ).
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,” Mackay said.
Harvard Health explains on its website here that PCR tests are highly accurate, with most false-positive results thought to be due to errors in performing the test.
Samadi did not immediately respond to Reuters’ request for comment on the misinformation resulting from his now-deleted tweet.
False. WHO did not release guidance saying that PCR tests for COVID-19 resulted in inflated positive results, rather sought to address accuracy issues with PCR tests not being conducted correctly.
This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .
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