Ed Verbeek schreef op 1 september 2020 13:04:
Hier een paar deskundigen aan het woord over de PCR-test, uit
www.nytimes.com/2020/08/29/health/cor... :
The usual diagnostic tests may simply be too sensitive
... adjust the cycle threshold used now to decide that a patient is infected. Most tests set the limit at 40, a few at 37. This means that you are positive for the coronavirus if the test process required up to 40 cycles, or 37, to detect the virus.
Tests with thresholds so high may detect not just live virus but also genetic fragments, leftovers from infection that pose no particular risk — akin to finding a hair in a room long after a person has left, Dr. Mina said [an epidemiologist at the Harvard T.H. Chan School of Public Health]
Any test with a cycle threshold above 35 is too sensitive, agreed Juliet Morrison, a virologist at the University of California, Riverside. “I’m shocked that people would think that 40 could represent a positive,” she said.
A more reasonable cutoff would be 30 to 35, she added. Dr. Mina said he would set the figure at 30, or even less. Those changes would mean the amount of genetic material in a patient’s sample would have to be 100-fold to 1,000-fold that of the current standard for the test to return a positive result — at least, one worth acting on.
.... the lab identified 794 positive tests, based on a threshold of 40 cycles.
With a cutoff of 35,
about half of those tests would no longer qualify as positive.
About 70 percent would no longer be judged positive if the cycles were limited to 30.In Massachusetts, from 85 to 90 percent of people who tested positive in July with a cycle threshold of 40 would have been deemed negative if the threshold were 30 cycles, 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