The main test used to diagnose coronavirus is so sensitive it could be picking up fragments of dead virus from old infections, scientists say.
Most people are infectious only for about a week, but could test positive weeks afterwards.
Researchers say this could be leading to an over-estimate of the current scale of the pandemic.
But some experts say it is uncertain how a reliable test can be produced that doesn’t risk missing cases.
Prof Carl Heneghan, one of the study’s authors, said instead of giving a “yes/no” result based on whether any virus is detected, tests should have a cut-off point so that very small amounts of virus do not trigger a positive result.
He believes the detection of traces of old virus could partly explain why the number of cases is rising while hospital admissions remain stable.
The University of Oxford’s Centre for Evidence-Based Medicine reviewed the evidence from 25 studies where virus specimens from positive tests were put in a petri dish to see whether they would grow.
This method of “viral culturing” can indicate whether the positive test has picked up active virus which can reproduce and spread, or just dead virus fragments which won’t grow in the lab, or in a person.
How is Covid diagnosed?
The PCR swab test – the standard diagnostic method – uses chemicals to amplify the virus’s genetic material so that it can be studied.
Your test sample has to go through a number of “cycles” in the lab before enough virus is recovered.
Just how many can indicate how much of the virus is there – whether it’s tiny fragments or lots of whole virus.
This in turn appears to be linked to how likely the virus is to be infectious – tests that have to go through more cycles are less likely to reproduce when cultured in the lab.