8/29/20, “Your Coronavirus Test Is Positive. Maybe It Shouldn’t Be.” NY Times, Apoorva Mandavilli (Print ed., Sunday, August 30, Sec. A, page 6, NY ed.)
“Some of the nation’s leading public health experts are raising a new concern…over coronavirus testing in the United States: The standard tests are diagnosing huge numbers of people who may be carrying relatively insignificant amounts of the virus.
The most widely used diagnostic test for the new coronavirus, called a PCR test, provides a simple yes-no answer to the question of whether a patient is infected….
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….
“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.”…
The Food and Drug Administration said in an emailed statement that it does not specify the cycle threshold ranges used to determine who is positive, and that “commercial manufacturers and laboratories set their own.”
The Centers for Disease Control and Prevention said it is examining the use of cycle threshold measures “for policy decisions.” The agency said it would need to collaborate with the F.D.A. and with device manufacturers to ensure the measures “can be used properly and with assurance that we know what they mean.”
The C.D.C.’s own calculations suggest that it is extremely difficult to detect any live virus in a sample above a threshold of 33 cycles.
Officials at some state labs said the C.D.C. had not asked them to note threshold values or to share them with contact-tracing organizations.
For example, North Carolina’s state lab uses the Thermo Fisher coronavirus test, which automatically classifies results based on a cutoff of 37 cycles. A spokeswoman for the lab said testers did not have access to the precise numbers….
“It’s just kind of mind-blowing to me that people are not recording the C.T. values from all these tests—that they’re just returning a positive or a negative,” said Angela Rasmussen, a virologist at Columbia University in New York….
Officials at the Wadsworth Center, New York’s state lab, have access to C.T. values from tests they have processed, and analyzed their numbers at The Times’s request. In July, 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….
The F.D.A. noted that people may have a
low viral load when they are newly infected. A test with less
sensitivity would miss these infections.
But that problem is easily solved, Dr. Mina said: “Test them again, six hours later or 15 hours later or whatever,” he said. A rapid test would find these patients quickly, even if it were less sensitive, because their viral loads would quickly rise….
Dr. Mina and other researchers are questioning the use of PCR tests as a frontline diagnostic tool.
People infected with the virus are most infectious from a day or two before symptoms appear till about five days after….
Highly sensitive PCR tests seemed like the best option for tracking the coronavirus at the start of the pandemic.”…
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