According to Jonathan Jarry, a bioscientist at McGill University, one of the most common misconceptions is that PCR tests are prone to a large number of false positives, and this often stems from a misunderstanding of how the test works. Office of Science and Society In Montreal.

Molecular biology is very complicated. Jarry said that it is easy for people to obtain some information, which is true in itself, but then will draw larger conclusions that are inaccurate.

“We are all vulnerable,” he said.

“I’m not accusing others of being stupid, stupid, or ignorant. These are prejudices that we all have to deal with…this is just part of being a person.”

Experts say that for some reason, PCR testing is often described as the “gold standard.” The National Human Genome Institute describes the technology as “One of the most important scientific advances In molecular biology. “

The main advantage, especially during a pandemic, is that the test is highly accurate, sensitive and fast.

PCR and the “old school” laboratory culture

PCR, which stands for “polymerase chain reaction”, is a method of repeatedly copying a piece of genetic material.

It is used to rapidly amplify small DNA fragments for more detailed study.

Modern laboratories use this method to test for a variety of different pathogens, including viruses, bacteria, fungi, and parasites.

Graham Tipples, director of medical science at the Alberta Precision Laboratories Public Health Laboratory, said that in the past, scientists had to rely more on “culture-based” methods, which included cultivating living pathogens in a controlled environment.

Tipples says that the “old school” approach is “very labor-intensive”-and very slow.

“It takes a day to a few days,” he said. “And it can be non-specific, which means you can’t be absolutely sure that it’s [particular] The viruses you detected because some of them look the same. “

In contrast, PCR tests look for genetic markers unique to specific pathogens, allowing scientists to be more confident about the results.

“The PCR test we use has been proven to be highly specific to the virus that causes COVID-19, SARS-CoV-2,” said Alberta Chief Health Officer, Dr. Deena Hinshaw, who recently held a special press conference to discuss Up Fighting the “myth” surrounding testing.

“It has no response to other viruses, even other coronaviruses,” she said.

Another advantage of PCR tests is their speed of execution.

“You can set up these types of tests on robotic systems to get very high throughput,” said Tipples, whose lab has conducted millions of tests in the past year.

He said the PCR test is also “extremely sensitive”, which means “you can detect a very small amount” of pathogens in a given sample.

Hinshaw said this is especially helpful for COVID-19 because it allows laboratories to detect viruses in people who are “in the early stages of the disease” and who do not have a large number of viruses in their bodies.

This also means that the PCR test will still detect a small amount of the virus at the end of a person’s disease, when their body has been fighting the virus most of the time.

Even a small number of dead viruses can be detected—experts say this fact has been distorted into false claims about flaws in the test.

Timothy Caulfield, chair of Canadian Health Law and Policy Research at the University of Alberta in Edmonton, said that the fact that some people deliberately misrepresent the working principle of PCR testing makes the problem even more complicated.

“I think one of the reasons it hasn’t died is because it fits the broader conspiracy theory so well that the entire pandemic is a hoax and the harm is exaggerated,” he said.

“Of course this is not true.”

Cycle threshold and context

A term often heard in social media circles is “cycle threshold” (CT), which refers to the number of cycles of replication required to detect the genetic material of a virus through a PCR test.

“The most common statement I see is that the CT value is too high, so the test is unreliable,” Caulfield said.

A high CT value does correspond to a small number of viruses, but it speaks more about the sample than the patient. The quality of the sample may vary depending on the sampling method (for example, nasal swabs and throat swabs), the technique of the health care worker sampling, the age of the sample, and other factors.

In the context of COVID-19, the main purpose of a positive PCR test is to determine whether the virus is present, not how much virus happens to be in a particular sample.

“In fact, different samples of the same person may result in different CT values,” Ontario Public Health Explained.

“If the DNA fragment cannot be replicated, then there is no virus in the sample, or the content is too low to detect it even with this very sensitive test.”

A positive test with a high CT value only indicates the presence of a small amount of virus in a given sample. This may be because the patient’s viral load is low, which may mean that they are at the beginning or end of the disease. Or it may be that the sample itself did not detect too many viruses.

“Any test needs to be used in the correct clinical and epidemiological context,” said Tipples of the Alberta Precision Laboratory.

“But you can’t ignore the fact that the test result is positive, and then report it as negative. It makes no sense at all.”

False negatives are more worrying

If so, experts are more worried about the possibility of false negatives The PCR test from COVID-19 depends largely on when the swab is taken relative to the time of infection.

A study last year It was discovered that the test never actually detected the virus on the first day of infection, and often reached its best state on the eighth day, but even so, the false negative rate can still be as high as 20%. After the 8th day, the study found that the false negative rate increased.

Epidemiologists incorporate this into their decision-making. This is why you hear about a “probable” case of COVID-19. When an exposed person is known to have symptoms but did not have a positive test, it is usually marked as a “probable” case.

Conversely, public health officials also considered the possibility that PCR tests could detect the dead virus in patients who have recovered and are no longer infectious.

Watch | New traveler quarantine rules for PCR testing:

Ottawa does not prohibit non-essential travel; it makes it as inconvenient and expensive as possible. Now, in addition to the existing requirements, returning travelers need to be quarantined at the hotel for three days at their own expense, which may cost at least US$2,000. 2:33

Hinshaw said: “This is why we don’t need a negative test before ending the quarantine.” “And why we focus on testing at the onset of symptoms and early after exposure to minimize this prolonged shedding. Impact.”

All of this shows that infectious diseases are complex, and PCR testing is part of the larger picture of COVID-19.

Experts in this field have been studying the complexities of all this all their lives, so Jarry of McGill University said that when the “armchair” scientists discover some information and turn it into wrong conclusions, it may be intimidating. Frustrated.

“There must be a certain degree of intellectual humility,” he said, “to participate in these ideas.”