Dr Clare Craig FRCPath The Covid Pimpernel: They seek it here, they seek it there, those testers seek it everywhere. Is it in Leicester, or in Torbay, that damned elusive RNA. On 7th September Public Health England announced a change to testing criteria. The change is designed to address the problem of false positives, people that test positive but do not in fact have COVID-19. The consequences of these false positives stretch far beyond the inconvenience to an individual who has to self-isolate unnecessarily. Counting false positives when a disease is not very prevalent can result in misleading, fear-mongering news articles and mistaken health policies. The impact of this change in testing criteria is yet to affect the case numbers being reported. The new criteria means some cases will need further assessment before being reported so those results will be delayed. Until Public Health England published this guidance, the working assumption was that false positive test results are irrelevant for testing carried out for COVID-19. This assumption is not true. However, in times of an epidemic, a few false positives does not make a material difference to the reported numbers of infected cases. When 30% of results are positive then 1% being false positive would be immaterial. When only 2% of your results are positive then 1% being false positive is important. ...