> In three sets of testing data that include cycle thresholds, compiled by officials in Massachusetts, New York and Nevada, up to 90 percent of people testing positive carried barely any virus, a review by The Times found ... One solution would be to adjust the cycle threshold used now to decide that a patient is infected. Most tests set the limit at 40, a few at 37 ... 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. 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.
When I used to do RT-PCR for research, 40 cycles would mean (to me) that I had nothing in my solution and I had to redo the experiment.
The precautionary principle (anti-scientific, because it assumes the worst even without evidence of such) is what drives such decisions, even if, according to Oxford's CEBM, there is little to no correlation between a PCR positive sample at 37+ cycles and actual presence of an infectious virus.
That is why antigen testing, although less sensitive, is important. A PCR test without symptoms does not tell if you have the virus you had the virus, and if you are infectious (assembled virus present in the respiratory tract) or not. Also, RNA in the throat can degrade slowly, and be present for a long time (this is not unique to this virus).
An antigen test will identify the virus proteins, and as such, a higher probability that a positive test detects an infectious virus.
https://www.nytimes.com/2020/08/29/health/coronavirus-testin...
> In three sets of testing data that include cycle thresholds, compiled by officials in Massachusetts, New York and Nevada, up to 90 percent of people testing positive carried barely any virus, a review by The Times found ... One solution would be to adjust the cycle threshold used now to decide that a patient is infected. Most tests set the limit at 40, a few at 37 ... 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. 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.