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Fingerstick tests may have given more false positives than previously thought
The accuracy of a rapid fingerstick antibody test for coronavirus may be considerably lower than previously stated, a new study suggests.
The results, published in the BMJ, suggest that if 10% of the people tested had previously been infected, about one in five positive test results would be false positives.
The researchers say these conclusions are in contrast to an earlier study, not yet peer-reviewed, which suggested that the tests did not return false positives.
These are completely different from the new successful piloted lateral flow antigen mass tests in Liverpool that can provide results in one hour.
The AbC-19 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 lab and is approved for use by healthcare professionals in the UK and EU.
Scientists, from Public Health England and the Universities of Bristol, Cambridge and Warwick, analyzed blood samples from 2,847 key workers (health personnel, firefighters and police) in England in June 2020.
Of these, 268 had a previously positive Covid-19 test result, thus they were known as positive, while the remaining 2,579 had an unknown prior infection status.
The scientists estimated that the sensitivity (ability to correctly identify a true positive sample) of the AbC-19 test was 92.5% based on PCR-confirmed cases, but considerably lower (84.7%) in people with a condition previous infection unknown prior to antibody test.
They suggest that this difference is likely due to the test being more sensitive when antibody levels are higher.
Since people with a positive PCR result tended to have more severe disease, it is likely that they would have produced more antibodies.
The researchers say that the lower figure of 84.7% is probably a more realistic estimate of the test’s sensitivity in the real world.
Therefore, if one million people were tested with AbC-19, of whom 10% had previously been infected with Covid-19, there would be 18,900 false positive results.
In general, they suggest that about one in five positive results would be wrong.
The study also found that trained laboratory personnel 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.
The researchers note a number of limitations to their study, such as the fact that the test was evaluated in a laboratory rather than the participants taking it themselves, which may have overestimated performance.
They also note that the study did not include many people over the age of 65, suggesting that further evaluation is needed in those for whom the risk of severe Covid-19 is substantially higher.
The UK government has placed an order for one million AbC-19 tests for research purposes to help build a picture of how the virus has spread across the country.
In a related editorial, Dipender Gill of Imperial College London and Mark Ponsford of Cardiff University wrote: “(This study) identifies notable limitations of the antibody test chosen by the UK government and provides good evidence that its specificity in a ‘real life’ setting is highly unlikely to be 100%. ”
However, the researchers found in laboratory settings that 97.9% of people who did not have Covid-19 had negative test results.
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