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.
Source: BBC News