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The accuracy of a rapid fingerstick antibody test for SARS-CoV-2, the virus responsible for covid-19 infection, may be considerably lower than previously suggested, a study published by finds. The BMJ.
The results suggest that if 10% of the people tested had already been infected, about 1 in 5 positive test results would be incorrect (false positive results).
These conclusions are in contrast to an earlier study (not yet peer-reviewed) suggesting that the test does not return false positive results.
The findings suggest that the test may offer a sufficient degree of precision for population surveillance studies, but laboratory confirmation of positive results will likely be needed if these tests are to be used to provide evidence of protection against the disease. virus.
The AbC-19TM rapid test uses a drop of blood from a finger prick to see if someone is likely to have previously been infected with SARS-CoV-2. It gives results in 20 minutes, without the need to go to a laboratory, and is approved for use by healthcare professionals in the UK and EU.
The latest research was commissioned by the Department of Health and Welfare and conducted by scientists from Public Health England and the Universities of Bristol, Cambridge and Warwick.
Scientists analyzed blood samples in a laboratory of 2,847 key workers – health personnel, firefighters and police – in England in June 2020.
Of these, 268 had a previous positive PCR result (positive polymerase chain reaction) making them “known positive” while the remaining 2,579 had an unknown previous infection status. An additional 1,995 pre-pandemic blood samples were also tested as “known negatives.”
Based on a series of analyzes, the researchers estimated that the specificity of the AbC-19 test (ability to correctly identify a true negative sample) was 97.9%, which means that 2.1% of people who did not they had a previous SARS-Cov-2 infection and were incorrectly positive
They estimated that the sensitivity of the AbC-19 test (ability to correctly identify a true positive sample) was 92.5% based on cases confirmed by PCR, but considerably lower (84.7%) in people with previous infection status. unknown prior to antibody test.
This difference is likely due to the test being more sensitive when antibody levels are higher, the researchers explain. Since people with a positive PCR result tended to have more severe disease, it is likely that they would have produced more antibodies.
They say the lower figure of 84.7% is probably a more realistic estimate of the test’s sensitivity in the real world, if people chose to take the test to find out their own previous infection status. This means that 15.3% of people with a previous SARS-CoV-2 infection would be lost.
Putting these findings in context, the researchers say that if 1 million people were tested with AbC-19, of whom 10% had previously been infected with SARS-CoV-2, there would be 18,900 false positive results. In general, about one in five positive results would be wrong.
They also found that trained laboratory personnel noted that the test result band was often weak and disagreed on whether the result was positive or negative for nearly 4% of the AbC-19 tests. This implies that the precision of the test could be even lower if the test were used at home by members of the public.
This is a large study, using data from people with known and unknown prior infection status, but the authors highlight some limitations.
For example, the test was evaluated in a laboratory, rather than having the participants perform the test themselves, which may have overestimated performance, and the study included few people over the age of 65, suggesting the need for a additional evaluation of the test in older ages when the risk of severe covid-19 is substantially higher.
Other lateral flow devices that detect antibodies to SARS-CoV-2 may also work less well at lower antibody concentrations; While this study did not investigate this, the authors note that their work “highlights the extent of the overestimation of the sensitivity of the SARS-CoV-2 antibody test in other studies in which the sensitivity has been estimated only from cases confirmed by PCR “.
The UK government has placed an order for one million tests of AbC-19 for research purposes, to help build a picture of how the virus has spread across the country.
In a linked editorial, Dipender Gill at Imperial College London and Mark Ponsford at Cardiff University say that this study “identifies notable limitations of the UK government-chosen antibody test and provides good evidence that its specificity in a “real life” setting it is highly unlikely to be 100%. “
They call for more work to clarify the relationship between circulating antibody levels and immunity to SARS-Cov-2, saying that “a clear message must be communicated to the public that positive results from these tests do not provide evidence of immunity.”
“Aside from limited surveillance to estimate the proportion of a population that has been infected, the widespread use of this assay in any other role could pose a considerable risk of harm,” they conclude.
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Peer reviewed? Yes (research); No (linked publisher)
Type of evidence: test precision study; Opinion
Topics: People
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