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In Massachusetts, 85-90 percent of people who tested positive in July with a cycle threshold of 40 would have been deemed negative with a Ct of 30, Dr. Mina said. “I would say that none of those people should be contact-traced, not one,” he said.
Other experts informed of these numbers were stunned. “I’m really shocked that it could be that high — the proportion of people with high C.T. value results,” said Dr. Ashish Jha, director of the Harvard Global Health Institute. “Boy, does it really change the way we need to be thinking about testing.”
The number of people with positive results who aren’t infectious is particularly concerning, said Scott Becker, executive director of the Association of Public Health Laboratories. “That worries me a lot, just because it’s so high,” he said, adding that the organization intended to meet with Dr. Mina to discuss the issue.
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The Lisbon court of Appeal recently upheld a lower court decision to declare unlawful the quarantining of four persons, one of whom had tested positive for COVID-19 using a PCR test. In doing so, the court made a number of observations about PCR tests. Citing a study by Jaafar et al, the court concluded “if someone is tested by PCR as positive when a threshold of 35 cycles or higher is used (as is the rule in most laboratories in Europe and the US), the probability that said person is infected is <3%, and the probability that said result is a false positive is 97%.”
The Jaafar et al study was published on 28 September 2020 in the journal Clinical Infectious Diseases. The French researchers found at a cycle threshold of 25, up to 70% of patients remained positive for COVID-19 in culture and that at a Ct of 30 this value dropped to 20%. At a Ct of 35, the false positive rate was a whopping 97%.
Their findings agreed with an earlier Canadian study in which confirmed COVID-19 infection was only observed with a Ct of 24 or less, and a symptom-onset-to-test time of under 8 days.