A****h
发帖数: 385 | 1 左派不是动不动就科学家如何如何，新冠措施一定要听科学家的吗？
现在有些科学家说新冠测试存在问题，故意放大新冠阳性数字。怎么看？新闻还是发表
在当地NBC的网站上。
我不是这方面专家，看下来，大致的理解就是：
新冠病毒很多人都有，但病毒数量不够则不能传染。现在的新冠测试，主要用PCR技术
，故意放大了信号，很多人病毒数量及其稀少，完全不能传染，一样也被算作新冠病人
了。
放大信号用的手段叫cycling，每cycle一遍信号就增强一些（指数增长还是线性？）。
大学实验室里面，cycle了34遍后，病毒不能培育，说明此时病毒没有传播性。但在很
多公立和私立的监测站里，cycle的遍数平均为35-40。这样，很多体内病毒数量不多不
够传染的无症状者，都被诊断为新冠阳性了。
Are NC's coronavirus case numbers inflated? Some scientists say so
Tags:
coronavirus testing, coronavirus, WRAL Investigates
Posted October 26, 2020 4:31 p.m. EDT
Updated October 26, 2020 6:26 p.m. EDT
By Cullen Browder, WRAL anchor/reporter
Raleigh, N.C. — North Carolinians have taken more than 3 million
coronavirus tests during the pandemic, with PCR testing accounting for 97
percent of the positive results.
Polymerase chain reaction, or PCR, tests involve rapidly making millions to
billions of copies of a specific DNA sample through a process called cycling
, amplifying it enough to study in detail. While the test is widely regarded
as accurate when it comes to positive or negative for coronavirus, some in
the scientific world say it provides an incomplete picture.
Dr. Carl Heneghan, director of the University of Oxford's Centre for
Evidence-Based Medicine, told The Spectator magazine in London, that the
black-and-white nature of the test is leading to an inflated number of cases.
"We have found RNA evidence in shedding for 78 days," Heneghan said. "When
you’re picking up asymptomatic people, you have no idea if they have an
active infection, or did they have it two months ago?"
For its coronavirus test, North Carolina's state public health lab cycles a
DNA sample up to 37 times before deciding whether it’s positive or negative
. LabCorp, the main private tester in the state, has a cycle threshold that
’s even higher, at 38.
"The more times you do that, the less virus you start out with," Heneghan
said.
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WRAL Investigates went through the specs of dozens of PCR tests that asked
for emergency approval from the U.S. Food and Drug Administration. The cycle
thresholds range from as low as 35 to as high as 40, depending on the maker.
"The higher that threshold goes, the dimmer the signal is – or translated
to virus, the lower amount of virus is present," said Thomas Denny, chief
operating officer of Duke University's Human Vaccine Institute, who heads up
the school's coronavirus testing program.
"If you think of a needle in a haystack, it’s very hard to find a needle in
a haystack," Denny said in explaining the cycling threshold concept. "But
if you can make 1,000 needles in that haystack or 1,500 needles in that
haystack, it’s easier to pick it out."
The higher thresholds raise questions when people are labeled as positive,
he said, noting the test could detect inactive virus.
But Scott Shone, director of the state's public health lab, said the debate
misses the point of the testing.
"These tests weren’t designed specifically to identify how much virus is
there, just whether it’s there or not," Shone said. "Trying to add
additional layers of analysis onto a test that wasn’t designed for that
purpose can be dangerous."
When labs report results to North Carolina, it’s only in the form of
positive or negative. Denny said he thinks health care providers could use
the cycle threshold results as one diagnostic tool, along with a patient’s
condition and known contacts. Someone with a high cycle threshold positive
result may not be an active case and may not need to quarantine, he said.
"I think knowing that is useful information, especially in the early days of
the disease where we are now," he said, adding that it’s better to have
testing that’s too sensitive because a false positive is better than a
false negative, which could lead to more spread.
Some researchers have tried to grow coronavirus from samples with cycle
thresholds higher than 34. But they were unsuccessful, meaning the virus
wasn’t active – yet the people who took the test were labeled as positive.
Dr. Michael Mina, an assistant professor of epidemiology at Harvard
University's T.H. Chan School of Public Health, has recommended cutting off
the cycle threshold closer to 30 to identify only those people who have
enough virus to spread. Lowering the cycle threshold would reduce cases by
80 to 90 percent, he said.
Heneghan said thresholds in the high 30s paint a false picture, causing
unnecessary quarantines and negative economic impacts,
"You can point a threshold level which says you are infectious, which is
about a million copies per [milliliter} in a sample, which is a cycle
threshold of about 25," he said.
That 25 threshold is exponentially lower than what most labs use to test for
the coronavirus. The FDA applications for LabCorp’s test, as well as the
ThermoFisher test used by the state lab, clearly say that, after a positive
result "other diagnostic information is needed to determine patient
infectious status."
In most cases, however, lab results are reported to the patient and local
health authorities without any follow-up examination. |
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