Coronavirus: tests ‘could be detecting dead viruses’

The main test used to diagnose the coronavirus is so sensitive that it could detect dead virus fragments from ancient infections, scientists say.

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

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

But some experts say it’s unclear how a reliable test can be produced that doesn’t risk missing cases.

Professor Carl Heneghan, one of the study’s authors, said that instead of giving a “yes / no” result based on whether any viruses are detected, the tests should have a cut-off point so that very small amounts of viruses do not generate a positive result. .

He believes that the detection of traces of ancient 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 reviewed the evidence from 25 studies in which test-positive virus samples were placed in a Petri dish to see if they grew.

This “viral culture” method can indicate whether the positive test has detected an active virus that can reproduce and spread, or simply fragments of dead virus that will not grow in the laboratory or in a person.

How is Covid diagnosed?

The PCR smear test, the standard diagnostic method, uses chemicals to amplify the genetic material of the virus so that it can be studied.

Your test sample has to go through a series of “cycles” in the laboratory before enough virus is recovered.

How many can indicate how many viruses there are, be it small fragments or many whole viruses.

This, in turn, appears to be related to how likely the virus is to be infectious – tests that have to go through more cycles are less likely to reproduce when grown in the lab.

False positive risk

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

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

But Professor Heneghan, the academic who discovered a quirk in the way deaths were being recorded, prompting Public Health England to reform its system, says evidence suggests that the infectivity of the coronavirus “appears to decrease after a week. “.

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

This could prevent people from testing positive due to a previous infection.

And Professor Heneghan said that would prevent people from being unnecessarily quarantined or tracked, and would give a better understanding of the current scale of the pandemic.

Public Health England agreed that viral cultures were a useful way of evaluating coronavirus test results and said it had recently conducted an analysis to this effect.

He said he was 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 ways, making it difficult to provide a variety of cutoffs.

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

“This review runs the risk of falsely correlating the difficulty of culturing Sars-CoV-2 from a patient sample with the likelihood that it will spread,” 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” about how long the virus remains infectious during the recovery period.

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

In Italy, which had its peak before the UK, “for several weeks we were overestimating cases” because people who contracted the infection several weeks before they were identified as positive.

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

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

“This is not evidence of infectivity,” he said. But the clinical consensus was that it was “highly unlikely that patients were infectious after day 10 of illness.”

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