A number can show how contagious a COVID-19 patient is. Should it be included in the test results? | Science


Positive coronavirus tests can also reveal an individual’s infectiousness.

Martin Burnett / AFP via Getty Image

By Robert F Services

ScienceCOVID-19 reporting is supported by the Pulwitzer Center and the Haising-Simmons Foundation.

Ever since the coronovirus pandemic began, battles have raged from trial to trial: which tests should be given, to whom, and how often? Now, epidemiologists and public health experts are opening a new debate. They say that testing centers should not only report if a person is positive, but also what is known as a cycle threshold (CT) value, which indicates how much an infected person infects the virus.

Advocates pointed to new research, saying that CT values ​​could help doctors characterize patients at high risk for critical illness. Recent findings also suggest that the number may help officers determine who is contagious and therefore should be isolated and their contacts tracked down. The CT value is an incomplete measure, acknowledging the advocates. But to add it to the test results “is one of the most pressing questions out there,” says Michael Mina, a physician and epidemiologist at Harvard University’s TH School of Public Health.

Standard tests identify SARS-COV-2 infections by isolating and amplifying viral RNA using a process called polymerase chain reaction (PCR), which takes several cycles of amplification to produce a detectable amount of RNA. it depends on. The CT value is the number of cycles required to spot the virus; PCR machines stop running at that point. If a positive signal is not seen after 37 to 40 cycles, the test is negative. But samples that turn out to be positive can start with a large amount of virus, for which the CT value provides an inverse measurement. A test that recorded positive results after 12 rounds, for a CT value of 12, started with more than 10 million times of viral genetic material as a sample with a value of 35 million CT.

But the same sample can give different CT values ​​on different testing machines, and different swabs from the same person can give different results. “The CT price is not a full scale,” says Marta Gaglia, a virologist at Tufts University. Meena says that many doctors have become cautious. “Physicians are cautious by nature,” Meena says. “They say, ‘If we can’t trust it, it’s not reliable.” “In an August letter Clinical infectious diseaseMembers of the College of American Pathologists urged caution in interpreting CT values.

Nevertheless, Meena, Gagalia, and others argue that knowing the CT value is high or low can be highly informative. “Even with all the flaws, the viral load can be extremely powerful to know,” Meena says.

Preliminary studies have shown that patients have a CT value below 30 in the first days of infection, and often below 20, indicating high levels of the virus; As the body clears the coronovirus, the CT value gradually increases. More recent studies have shown that a higher viral load can profoundly affect a person’s contagious disease and reflects the severity of the disease.

In a study published this week Clinical infectious diseaseResearchers, led by Bernhard La Scola, an infectious disease specialist of IHU-Méditerranée infection, examined 3790 positive samples, with known CT values ​​to determine if they harmed viable viruses, indicating patients were infectious. . La Scola and his colleagues found that 70% of samples with 25 or more CT values ​​could be cultivated, compared to 35% of cases with CT values ​​greater than 35%. “It is fair to say that a higher viral load is associated with being infectious,” says Monica Gandhi, an infectious disease specialist at the University of California, San Francisco.

In contrast, people often test positive for weeks or even months after recovering, but have high CT values, suggesting that PCR has identified genetic material from non-infectious viral debris. Current guidelines from the Centers for Disease Control and Prevention and the World Health Organization, which ask patients to isolate themselves for 10 days after the onset of symptoms, recognize that they are unlikely to be contagious after that period Huh. But Meena and others say recent findings also suggest that a patient who has undergone multiple tests with high CT values ​​is more likely to be at the tail end of their infection and to isolate themselves is not needed. He said that the contactors should try their efforts on the basis of CT values. “If 100 files come to my desk [as a contact tracer]”I would prefer viral loads first, because they are the most infectious,” says Meena.

Meena says that the widespread penetration of CT values ​​can also help reduce epidemiologists’ outbreaks. If researchers see many low CT values, they may conclude that an outbreak is expanding. But if almost all CT values ​​are high, outbreaks are likely. “We need to stop people thinking about positive or negative, and ask how positive?” Says Meena.

The CT value can also help clinicians put the most flag at risk of serious illness and death. A June report by Well Cornell Medicine researchers found that among 678 hospitalized patients, 35% of those with a CT value of 25 or less died, while a CT value of 17.6% was 25 to 30 and a 6.2% CT value. Was with Above 30. In August, researchers in Brazil found that among 875 patients, those with a CT value of 25 or below were more likely to die of serious illness or die.

Gandhi agrees that access to CT values ​​can help clinicians identify those at high risk for developing symptoms. Nevertheless, he and others note that a high viral load does not necessarily lead to disease; Some 40% of people who contract SARS-CoV-2 remain healthy, while they have an equal amount of viruses that fall ill. “As a physician, having a CT value is not the only thing I will use to diagnose and track patients,” says epidemiologist Chanu Ray of the hospital at Brigham and Women’s Hospital. “But I still find it helpful.”

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