A quarter of people who test positive for antibodies to coronavirus would still be at risk from Covid-19, even if the test was 98 per cent accurate, a group of scientists advising the government has warned.
The UK medicines regulator has set a minimum accuracy of 98 per cent for coronavirus antibody tests. If you test 1,000 people at this level of accuracy, assuming 5 per cent of the population are thought to have antibodies, then 68 people would be told they have antibodies. But 19 of those people, or 28 per cent, would not actually have the antibodies. This would leave them at risk of future infection, the Scientific Advisory Group for Emergencies (Sage) concluded in documents revealed today, as first reported by the Independent.
The problem persists, although to a lesser extent, if you assume 10 per cent of the population have antibodies (16 per cent will get a false positive result), or even if the test is 99 per cent accurate. At this level of accuracy, a 5 per cent prevalence of the virus means 16 per cent of those testing positive for antibodies would not actually be protected.
The scientists said this could have potentially dangerous consequences. People who tested positive might assume they are immune and return to work, potentially catching and spreading the virus, which would have “more severe consequences for frontline health and social care workers”, they said. “Although in theory, staff should not attend work with fever even outside of a pandemic, in practice presenteeism can be high, particularly among healthcare workers.”
People with a false positive antibody test result might also stop following social distancing rules, and might volunteer for tasks that put them at higher risk of exposure, they said.