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new study Rapid Covid-19 antibody blood tests ‘less accurate’ than first believed

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Express COVID-19 test for IgM and IgG antibodies to novel coronavirus SARS-CoV-2, Covid-19. Nurse hand in nitril glove collects patient blood with disposable pipette. Gren mint turquoise background.

Express COVID-19 test for IgM and IgG antibodies to novel coronavirus SARS-CoV-2, Covid-19. Nurse hand in nitril glove collects patient blood with disposable pipette. Gren mint turquoise background.

Express COVID-19 test for IgM and IgG antibodies to novel coronavirus SARS-CoV-2, Covid-19. Nurse hand in nitril glove collects patient blood with disposable pipette. Gren mint turquoise background.

The accuracy of a rapid finger-prick antibody test to find out if someone had coronavirus may be considerably lower than previously suggested, a new study reveals today.

These tests try to find out if someone had the virus in the past and they are useful for seeing how Covid-19 is spreading.

The results suggest that if 10pc of people given the test had previously been infected, around one in five positive test results would be incorrect.

These conclusions contrast with an earlier study suggesting that the test gives no false positive results.

The findings suggest the test can deliver a sufficient degree of accuracy for surveillance studies of the population, but laboratory confirmation of positive results is likely to be needed if these tests are to be used to provide evidence of protection from the virus.

The AbC-19TM Rapid Test uses a drop of blood from a finger-prick to see if it’s likely that someone has previously been infected with the virus.

It gives results in 20 minutes without the need to go to a laboratory, and is approved for use by health professionals in the EU, including Ireland.

The latest research in the British Medical Journal was conducted by scientists from Public Health England and the Universities of Bristol, Cambridge and Warwick.

Scientists tested blood samples in a laboratory from 2,847 key workers, healthcare, fire and police staff, in England in June 2020.

Of these, 268 had a previous PCR (positive polymerase-chain reaction) positive result so were “known positives” while the remaining 2,579 had unknown previous infection status. A further 1,995 pre-pandemic blood samples were also tested as “known negatives”.

Based on a series of analyses, the researchers estimated the specificity of the AbC-19 test – its ability to correctly identify a true negative sample – to be 97.9pc. It means 2.1pc of people who did not have a previous infection incorrectly tested positive.

They estimated the sensitivity of the AbC-19 test – the ability to correctly identify a true positive sample – to be 92.5pc based on PCR confirmed cases, but considerably lower – at 84.7pc – in people with unknown previous infection status prior to antibody testing.

This difference is probably due to the test being more sensitive when antibody levels are higher, explain the researchers. As 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 84.7pc is probably a more realistic estimate of test sensitivity in the real world, if people were to choose to take the test to find out their own previous infection status. This means that 15.3pc of people with a previous virus infection would be missed.

Putting these findings into context, the researchers say that if one million people were tested with AbC-19, of whom 10pc had been previously infected, there would be 18,900 false positive results. Overall, about one in five positive results would be wrong.

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