Carl Heneghan also said that the potential for false-positives to drive the increase in community cases was “substantial”.
“Inaccuracies” and “poor interpretation” of testing data could lead to mistakes about imposing coronavirus restrictions, a scientist has warned.
Carl's comments come after new restrictions were imposed in parts of north-west England due to a rise in Covid-19 transmission rates.
The professor of evidence-based medicine at the University of Oxford said that the potential for false-positives – people without the disease who test positive – to drive the increase in community cases was “substantial”.
Writing on the Centre for Evidence-Based Medicine (CEBM) website on Sunday, Prof Heneghan said it was “essential” to adjust the number of cases against the number of tests being carried out.
He said that the data from tests carried out in laboratories, known as pillar one, and those in the community, pillar two, in England from July showed a trend of increasing case numbers from about 500 a day to nearly 750.
Prof Heneghan said that on July 1 the seven-day moving average of testing was 41,109 for pillar one and 43,161 in pillar two.
But by July 31, the pillar one seven-day average for testing had increased by 20% to 49,543 while the pillar two had risen by 82% to 78,522 tests, he said.
Prof Heneghan said that when case numbers are adjusted for the number of tests done and standardised to per 100,000 tests, it showed that while pillar one was trending down, pillar two was flatlining.
He said that the increase in the number of cases detected was likely due to the increase in testing in pillar two, with Leicester and Oldham – two areas which have been subject to further restrictions due to increased cases – seeing “significant” increases in testing over a short time.
He added: “It is essential to adjust for the number of tests being done.
“Leicester and Oldham have seen significant increases in testing in a short time.
“Leicester, for example, in the first two weeks of July, did more tests than anywhere else in England: 15,122 tests completed in the two weeks up to July 13.
“The potential for false-positives (those people without the disease who test positive) to drive the increase in community (pillar two) cases is substantial, particularly because the accuracy of the test and the detection of viable viruses within a community setting is unclear.
“Standardising cases per tests done, and aligning the counts in different datasets to provide the same numbers, will allow a better understanding of whether cases are going up or down.
“Inaccuracies in the data and poor interpretation will often lead to errors in decisions about imposing restrictions, particularly if these decisions are done in haste and the interpretation does not account for fluctuations in the rates of testing.”