Covid Positivity Rates Drop 60-90% When PCR Tests are Adjusted

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Health experts are saying tests for Covid19 tests are too sensitive and should be adjusted to rule out individuals who insignificant traces of the virus in their system.

Current test’s thresholds are so high, they detect people with the live virus who are contagious, as well as individuals with a few genetic fragments left over from a past infection that no longer pose a risk.

Reverse transcriptase quantitative polymerase chain reaction (PCR) tests use nasal swabs to collect RNA from a person’s nasal cavity. The RNA is reverse transcribed into DNA, and then amplified through 40 or more cycles. The result is then reported as a simple “yes” or “no” to the question of whether a person is infected with Covid19.

But tests with cycle thresholds (CT) above 35 are too sensitive, health officials say. “I’m shocked that people would think that 40 [cycles] could represent a positive,” says Juliet Morrison, PhD, a virologist at the University of California, Riverside. A more reasonable cutoff would be 30-35, she says.

Michael Mina, MD, an epidemiologist at the Harvard T.H. Chan School of Public Health, would set the figure at 30, or even less.

The CDC itself has said it is extremely difficult to detect any live virus in a sample that has been put through 33 CT’s or above.

“We’ve been using one type of data for everything, and that is just plus or minus—that’s all,” Dr. Mina said. “We’re using that for clinical diagnostics, for public health, for policy decision-making.” But “yes” or “no” isn’t good enough, he adds. It’s the amount of virus that should dictate diagnosis and next steps. “It’s really irresponsible, I think, to forgo the recognition that this is a quantitative issue,” he adds.

Officials at the Wadsworth Center, New York’s state lab, have access to CT values from tests they have processed. At The New York Times request, they re-analyzed their numbers. In July, the lab identified 872 positive tests, based on a threshold of 40 cycles. With a cutoff of 35 cycles, about 43% of those tests would no longer be considered positive. 63% percent would no longer be judged positive if the CT threshold was 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.

“I’m really shocked that it could be that high—the proportion of people with high CT value results,” said Ashish Jha, MD, director of the Harvard Global Health Institute. “Boy, does it really change the way we need to be thinking about testing.”

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