Coronavirus: Antibody tests would put up to quarter of those told they were immune at risk of infection, government advisers warn

Scientists have warned the government that even highly accurate antibody tests for coronavirus could leave more than a quarter of people who were told they were immune at risk of infection.

New documents published today reveal the problem exists even for antibody tests that are 98 per cent accurate. Even if the tests are 99 per cent accurate almost 10 per cent of people who were told they were immune would be put at risk.

The conclusions, considered by the government’s Scientific Advisory Group for Emergencies, or Sage, suggest the much-anticipated development of home antibody kits may not be the route to exiting the coronavirus lockdown many had hoped.

The problem comes from the number of patients who may receive a false result even though the tests are so accurate.

The Sage paper says the performance of the tests depends on how many people who actually have antibodies test positive, this is called the sensitivity, and how many people without antibodies test negative, which is described as the specificity.

Alongside this is the spread of the disease within society and the number of people who actually have antibodies – the prevalence.

The Sage document said for a test which is more than 98 per cent accurate and where 5 per cent of the population are thought to have antibodies, a test of 1,000 people would lead to 68 being told they had had the virus and were immune.

Of these, 19 people, or 28 per cent would not have antibodies and would be put at risk of infection if allowed to go back to work or mix in public with those carrying the virus. At least one person out of 1,000 would be wrongly told they had not had the virus and would be forced to stay in lockdown.

If the spread of the virus increases to 10 per cent of the population, the results mean 116 people out of 1,000 would be told they were immune, but of those 18, or 16 per cent, would not actually have antibodies and susceptible to infection.

The problem persists even if the test is 99 per cent accurate. With a 5 per cent prevalence of the virus, 16 per cent of those who test positive for antibodies would be incorrect, with numbers falling to 8 per cent if the virus has infected 10 per cent of the population.

The Medicines and Healthcare products Regulatory Agency, which regulates medical devices, has set a minimum standard of 98 per cent accuracy for antibody test kits.

The paper warns there could be behavioural consequences as a result of widespread use of antibody testing with people who test positive and believe they are immune to becoming infected and spreading the virus.

It said: “This may occur in all population groups, but would have more severe consequences for frontline health and social care workers, where failure to self-isolate may result in staff members infecting vulnerable patients and coworkers. 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.”

The scientists also warned people may stop following rules around hygiene and social distancing if they believe they are immune and could volunteer for tasks that put them at higher risk such as interacting with the public in their work.

Similarly, people told they have not had the virus may isolate themselves more and could face discrimination by their employers.

It said: “This might include not permitting those [who test] antibody negative to return to work, or only taking on new staff with antibody positive test results. Work may also be allocated among employees based on test status with, for example, customer-facing work being allocated to those who have tested antibody positive.

“In some circumstances, this may be appropriate, but in others, this might constitute adverse discrimination.”

The advisers also warned against the risk of a black market in antibody testing warning: “If a test result is a requirement for a resumption of work, a range of strategies to ‘game’ the system may arise. These include people deliberately seeking out infection or attempting to purchase a fake test result, commercial organisations selling unapproved tests, or approved tests becoming available through private organisations at prices that make them unavailable to most.”