Use them at the wrong time and they don’t work’: Antibody tests are only accurate when used between three and four weeks after you’ve had Covid-19, major review warns
Coronavirus antibody tests are only known to be accurate between three and four weeks after someone has had Covid-19, a scientific review has found.
And it said none of the tests that are available now are good enough to use outside of a hospital because they give too many false results.
They may also not work on people who have only had a mild illness, but researchers can’t be sure because almost all studies have been done on people who were so badly ill that they were in hospital.
The 300-page independent scientific review, led by the Cochrane institute and the University of Birmingham, analysed at least 54 studies of antibody tests.
The tests examine people’s blood to look for antibodies – substances made by the immune system – that indicate whether they have had Covid-19 in the past.
In the UK the tests, which Boris Johnson once promised people would be able to take at home, are only available in hospitals or as part of government surveys.
The accuracy of them is a huge sticking point – they can detect less than 30 per cent of positive results if used at the wrong time.
And scientists still aren’t really sure what they mean. In usual medicine, the presence of certain types of antibodies means someone is almost guaranteed not to get an illness again – but there is still no proof people can’t get Covid-19 more than once.
Antibody tests examine someone’s blood to look for signs that they have been infected with Covid-19 in the past. In the UK they are only available in hospitals or as part of government surveys (Pictured: A blood sample collected by West Midlands Ambulance Service)
Professor Jon Deeks, a medical tests expert at the University of Birmingham, was one of the leading scientists on the international review.
He said: ‘We’ve analyzed all available data from around the globe – discovering clear patterns telling us that timing is vital in using these tests.
‘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 hospitalized patients, so we don’t really know how accurately they identify COVID-19 in people with mild or no symptoms, or tested more than five weeks after symptoms started.’
The Cochrane review found that the third and fourth week after someone has been infected with the coronavirus are the optimum time to use them.
Too soon, and they are inaccurate, but too late and their accuracy is completely unknown.
If someone took one of the blood tests within two weeks of developing symptoms, studies found, only seven out of 10 Covid-positive people would receive a positive result (70 per cent test sensitivity).
Between 15 and 35 days after symptoms, this accuracy increased to more than 90 per cent, on average.
For patients who had symptoms 35 days ago or longer, there were ‘insufficient studies’ to estimate how well the tests could work.
The researchers looked at studies evaluating 27 different types of antibody test – out of approximately 200 on the market – and said there was not enough data available to show whether lab-based tests were definitely better than hand-held ones.
Dr Jac Dinnes, a Birmingham researcher who worked on the review, said the studies that had been done on antibody tests so far were not of high quality.
She said: ‘The design, execution and reporting of studies of the accuracy of Covid-19 tests requires considerable improvement.
‘Studies must report data broken down by time since onset of symptoms.
‘Action is needed to ensure that all results of test evaluations are available in the public domain to prevent selective reporting. This is a fast-moving field and we plan to update this review regularly as more studies are published.’