Coronavirus antibody tests that can be done at home would get the results wrong a third of the time, a damning study has found.
A review of research found that rapid ‘have you had it’ coronavirus kits, which screen the blood to look for signs of past infection, have major flaws.
They correctly identify someone who has had the disease only 66 per cent of the time, on average – and this could drop as low as 49 per cent in some cases.
Officials in the UK have refused to offer antibody tests to the public because they fear people will stop following social distancing rules if they think they’re immune to Covid-19. In reality, there is no proof that people cannot get the illness twice.
The study in the British Medical Journal found that lab-based tests were more accurate but still not perfect, correctly identifying positive results between 84 and 97.8 per cent of the time.
The findings, based on 40 studies from around the world – most of them in China – suggest the tests are not ready for mass use.
And the team of experts said antibody testing should be halted until they are improved.
However, academics added it’s too early to rule out the tests as a useful pathway out of the pandemic, because the conclusions are unreliable.
A review of research found that rapid ‘have you had it’ coronavirus kits, which give a result on the spot, correctly identify someone has had the disease just 66 per cent of the time. This compares with laboratory-read antibody tests – which can correctly identify someone who has had Covid-19 at least 84 per cent of the time
The hope of antibody tests are that they can show that someone has previously been infected with SARS-CoV-2, the virus that causes Covid-19.
There is a further assumption that a positive test result would indicate that person is immune from re-infection and could possibly go back to work.
But this possibility has been largely discredited already, for example on the grounds that we still do not know how immune people are, if they have antibodies to the virus, and we do not know how long any immunity might last.
The latest review of evidence indicates antibody tests are not yet good enough to detect Covid-19 antibodies in the blood.
A team of international researchers led by the McGill University Health Centre in Montreal, Canada, looked at 40 studies between January 1 and April 30 that measured the accuracy of a Covid-19 antibody test.
Some 70 per cent were from China. The rest were from the UK, US, Denmark, Spain, Sweden, Japan and Germany.
There are two ways that an antibody test can give a wrong result.
It could say the blood sample contains antibodies when in fact the person was never infected – a false positive. Or it could declare that there are no antibodies when in fact the person was previously infected – a false negative.
Sensitivity of an antibody test measures the percentage of people who are correctly identified as having a disease.
Specificity measures the percentage of people who are correctly identified as not having a disease.
Dr Mayara Lisboa Bastos and colleagues found the sensitivity of antibody tests broadly ranged from 66 per cent to 97.8 per cent depending on the type of test used.
This means that between 2.2 and 34 per cent of patients who have had Covid-19 would be missed and wrongly told they had never had the illness.
Lateral flow immunoassays (LFIA), the method that would be used for home test kits, has the lowest sensitivity – between 49 and 79 per cent – meaning up to half of people who have had Covid-19 would not be spotted.
The average sensitivity of laboratory tests – known as ELISAs and CLIAs – was 84.3 per cent and 97.8 per cent, respectively.
All types of antibody tests would work using a blood sample from either a finger prick or a vein.
But LFIA tests can give a result on the spot, like a pregnancy test, while ELISA and CLIAs must be sent to a lab and carried out by technicians using specialised equipment.
When specificity results of the antibody tests in each study were pooled together, they ranged from 96.6 to 99.7 per cent, meaning that between 3.4 and 0.3 per cent of patients would be wrongly identified as having had Covid-19.
This could be dangerous if those people believe they have already had the infection and are safe to stop taking preventative measures when they are in fact still at risk.
Once again, the lab tests were more specific than the LFIA test.
Based on the results, the authors explain that, if an LFIA test is applied to a population where 10 per cent of people have had Covid-19, for every 1,000 people tested:
- 31 who never had the disease will be incorrectly told they are immune.
- 34 people who had Covid-19 will be incorrectly told that they were never infected.
If five per cent of people had been infected, which is closest to the most recent estimate for England from the Office for National Statistics infection survey:
- 32 would be incorrectly classified as infected.
- 17 would be incorrectly classified as uninfected.
The researchers said: ‘Currently, available evidence does not support the continued use of existing point-of-care serological tests.’
LFIA tests can give a result on the spot, like a pregnancy test, and are highly sought for. Ministers promised Brits would be able to get pregnancy-test style blood kits to tell them whether they’ve ever had the virus back in March. But none have proved accurate enough
Studies also showed antibody tests were most sensitive when they were conducted at least three weeks after symptoms started.
They were sensitive a maximum of 98.9 per cent of the time compared with within the first week, a maximum of 50.3 per cent of the time.
The findings back another review of a similar size, looking at 54 studies on a range of antibody tests, published last week.
The 300-page independent review, led by the Cochrane institute and the University of Birmingham, said antibody tests can detect fewer than 30 per cent of positive results if used too early.
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.
It showed that antibody tests are only accurate between three and four weeks after someone has had Covid-19.
They may also not work on people who have only had a mild illness, but researchers admitted they can’t be sure because almost all of the studies have been carried out on patients who were so badly ill they were hospitalised.
But Professor Jon Deeks, who led the Cochrane review, said antibody testing should not be ruled out yet.
Commenting on today’s findings, he said: ‘Point of care tests have benefits in terms of accessibility and speed over laboratory tests, which may make them preferable in some settings, even if they do not achieve exactly the same level of accuracy.
‘Their development and evaluation is of importance and should continue.’
Scientists have said the reviews come to similar conclusions, but are not entirely reliable.
Dr Alexander Edwards, an associate professor in biomedical technology, University of Reading, said: ‘By collating together so many studies, we are starting to see a picture emerge, but it’s still a sketch and there are still important gaps where it’s not yet possible to even guess what the picture shows.’
Kevin McConway, emeritus professor of applied statistics, The Open University, said: ‘If these reviews are based on studies that are not always very good science, it must be the case that the conclusions from the reviews cannot be as clear as would be ideal.
‘In fact that finding [of Dr Bastos’ review] is not entirely robust, statistically, and it remains plausible that the difference between the LFIA tests and others could perhaps, just about, be explainable by random variation.’
The UK Prime Minister Boris Johnson has described antibody tests as ‘game changing’ in its response to the pandemic.
Ministers promised Brits would be able to get pregnancy-test style blood kits to tell them whether they’ve ever had the virus back in March.
But three months down the line the tests still haven’t materialised for the public, as officials have still yet to approve any.
In a big step forward, health officials finally approved two lab-based tests made by pharma giants Roche and Abbott last month and bought 10million of the kits.
But they are only available for health and social care staff and results need to be read in the lab.