Antibody checks may only work up to 5 weeks after first symptoms

VIRUS antibody tests may not work unless they are done between two and five weeks after patients get infected, a major study says.

Experts said ‘ timing is everything’ with the blood tests, designed to detect antibodies showing whether someone has previously had the disease.

The review of 54 studies by British scientists will raise further doubts over whether they can be relied upon to say for certain if a person has been infected.

The study, published yesterday, found antibody tests are only 70 per cent accurate if performed within two weeks of the first symptoms. This accuracy rises to more than 90 per cent, on average, between 15 and 35 days after symptoms developed. But for patients who first displayed symptoms more than five weeks earlier, there was ‘insufficient’ evidence to show how well the tests could work. The tests may also fail to detect cases where people have been infected with coronavirus but experienced only mild symptoms or none at all, scientists found.

The 300-page independent scientific review, led by the Cochrane institute and the University of Birmingham, will add to questions over the over the usefulness of the

UK’s antibody testing strategy. Last month the Government bought 10million test kits from Abbott and Roche, currently available only to frontline health and social care staff. Boris Johnson has called them ‘game changing’.

Professor Jon Deeks of the University of Birmingham, who led the review, said: ‘Use them at the wrong time and they don’t work.

While these first Covid-19 antibody tests show potential, particularly when used two or three weeks after the onset of symptoms, the data are nearly all from hospitalised patients, so we don’t really know how accurately they identify Covid19 in people with mild or no symptoms, or those tested more than five weeks after symptoms started. If you use an antibody test too soon it’s got a higher chance of missing the disease… we just don’t know at the other end of the time spectrum how late we can use it.’

Antibodies are produced by the immune system to remember how to fight off a particular illness. But the presence of coronavirus antibodies doesn’t guarantee someone will not get the illness again.

In a letter to the British Medical Journal yesterday, senior doctors raised questions about how good the kits from Abbott and Roche are and argued there is no valid clinical reason for large-scale testing.

They warned: ‘The [test] is being rolled out at an unprecedented pace and scale without adequate assessment.’

The experts added that because it is not yet known whether antibodies confer immunity, a positive antibody test currently has no practical benefit, even for healthcare workers.

Trial data for the Abbott and Roche tests was not included in yesterday’s Cochrane review.