New York Times 29 augustus 2020
Dr. Michael Mina, an epidemiologist at the Harvard T.H. Chan School of Public Health.
“The PCR test amplifies genetic matter from the virus in cycles; the fewer cycles
required, the greater the amount of virus, or viral load, in the sample.
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.
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.