Coronavirus: tests “could detect a dead virus”

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The main test used to diagnose the coronavirus is so sensitive it could pick up fragments of dead viruses from old infections, scientists say.

Most people are only contagious for about a week, but could test positive weeks later.

Researchers say this could lead to an overestimation of the current scale of the pandemic.

But some experts say it’s not sure how to produce a reliable test that isn’t likely to miss cases.

Professor Carl Heneghan, one of the study’s authors, said that instead of giving a ‘yes / no’ result based on detecting a virus, the tests should have a cut-off point so that very small amounts of virus do not trigger a positive result. .

He believes that the detection of traces of old viruses could partly explain why the number of cases is increasing while hospital admissions remain stable.

The Center for Evidence-Based Medicine at the University of Oxford looked at evidence from 25 studies where virus samples from positive tests were placed in a Petri dish to see if they would grow.

This “viral culture” method can tell if the positive test has detected an active virus that can reproduce and spread, or just fragments of dead virus that will not develop in the lab or in a person.

How is Covid diagnosed?

The swab PCR test – the standard diagnostic method – uses chemicals to amplify the virus’s genetic material so that it can be studied.

Your test sample must go through a number of laboratory “cycles” before enough virus is collected.

The exact number can indicate how much virus is present – be it tiny fragments or many whole viruses.

This in turn appears to be related to the likelihood of the virus being infectious – tests that have to go through more cycles are less likely to recur when grown in the lab.

False positive risk

But when you take a coronavirus test, you get a “yes” or “no” answer. There is no indication of how much virus is in the sample or how likely it is to be an active infection.

A person shedding a large amount of active virus and a person with remaining fragments of an infection that has already been cleared would receive the same test result – positive.

But Professor Heneghan, the scholar who spotted a quirk in the way deaths were recorded that led Public Health England to reform its system, says evidence suggests that the coronavirus’ infectivity appears to wane after about one week ”.

He added that while it would not be possible to check every test to see if there was an active virus, the likelihood of false positive results could be reduced if scientists could determine where the cutoff point should be.

This could prevent people from getting a positive result based on an old infection.

And Professor Heneghan said it would prevent people from quarantining or being searched unnecessarily, and give a better understanding of the current scale of the pandemic.

Public Health England agreed viral cultures were a useful way to assess coronavirus test results and said it had recently undertaken a scan in this direction.

He said he’s working with labs to reduce the risk of false positives, including looking at where the “cycle threshold,” or cutoff point, should be set.

But he said there were many different test kits in use, with different thresholds and reading modes, making it difficult to provide a range of cutoff points.

But Professor Ben Neuman of the University of Reading said growing the virus from a patient sample was “not trivial.”

“This review runs the risk of falsely correlating the difficulty of growing Sars-CoV-2 from a patient sample, with the likelihood of it spreading,” he said.

Professor Francesco Venturelli, an epidemiologist from the Italian region of Emilia-Romagna, which was hit hard by the virus in March, said there was “not enough certainty” on how long the virus remains infectious during the recovery period.

Some studies based on viral cultures have reported that about 10% of patients still have viable virus after eight days, he said.

In Italy, which had its heyday earlier than the UK, “for several weeks we overestimated cases” due to people who contracted the infection several weeks before being identified as positive.

But, as you move away from the peak, this phenomenon decreases.

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Professor Peter Openshaw of Imperial College London said PCR was a “very sensitive method of detecting residual viral genetic material”.

“It’s not proof of infectivity,” he said. But the clinical consensus was that patients were “very unlikely to be infectious beyond day 10 of illness.”

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