Coronavirus Cases Plummet When PCR Tests Are Limited to Under 36 Amplification Cycles

By Barbara Caceres

Published September 29, 2020 | Medicine, Public Health

Health experts now say that PCR testing for SARS-CoV-2, the virus associated with the illness COVID-19, is too sensitive and needs to be adjusted to rule out people who have insignificant amounts of the virus in their system.1 The test’s threshold is so high that it detects people with the live virus as well as those with a few genetic fragments left over from a past infection that no longer poses a risk. It’s like finding a hair in a room after a person left it, says Michael Mina, MD, an epidemiologist at the Harvard T.H. Chan School of Public Health.2

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 New York Times found.3

Manufacturers and Labs Set Criteria for Positive COVID-19 Test Results

The reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) test used to identify those people infected with the SARS-CoV-2 virus uses a nasal swab to collect RNA from deep within the nasal cavity of the individual being tested. The RNA is reverse transcribed into DNA and amplified through 40 or more cycles, or until virus is detected.4 The result is reported as a simple “yes” or “no” answer to the question of whether someone is infected.

The U.S. Food and Drug Administration (FDA) officials state they do not specify the cycle threshold ranges used to determine who is positive, and that commercial manufacturers and laboratories set their own threshold ranges.5

PCR Test Threshold for COVID-19 Positivity Is Too Sensitive

Any test with a cycle threshold (CT) above 35 is too sensitive, says Juliet Morrison, PhD, a virologist at the University of California, Riverside. “I’m shocked that people would think that 40 [cycles] could represent a positive.” 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 worth acting on.6

The CDC’s own calculations suggest that it is extremely difficult to detect any live virus in a sample above a threshold of 33 cycles.7

“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 added. It’s the amount of virus that should dictate the infected patient’s next steps. “It’s really irresponsible, I think, to forgo the recognition that this is a quantitative issue,” Dr. Mina said.8

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.9

SARS-CoV-2 Positive Case Numbers Drop When Cycle Threshold is Adjusted, Removing Need for Contact Tracing

Officials at the Wadsworth Center, New York’s state lab, have access to CT values from tests they have processed, and analyzed their numbers at The Times’s request. In July, the lab identified 872 positive tests, based on a threshold of 40 cycles. With a cutoff of 35 cycles, about 43 percent of those tests would no longer qualify as positive. About 63 percent would no longer be judged positive if the cycles were limited to 30.



Coronavirus Cases Plummet When PCR Tests Are Adjusted — Madison Area Lyme Support Group

2 thoughts on “Coronavirus Cases Plummet When PCR Tests Are Limited to Under 36 Amplification Cycles

  1. The problem with using the RT-PCR test to diagnose COVID19 is that it doesn’t measure live virus – but only the presence of RNA particles. Because the quantities of RNA (converted to DNA prior to amplification) are so minute, they must be amplified (allowed to repeatedly duplicate). With too many amplifications, random RNA particles can be misidentified as COVID19. CDC data suggests the cutoff should be 33 amplifications. Other labs can detect live virus where a RT-PCR is positive with 35 amplifications.

    Unfortunately most labs are reporting a simple “positive” or “negative” on their RT-PCR tests – when they should be reporting the number of amplifications required to achieve that result.


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