One in five who undergo rapid Covid testing could be incorrectly told they have virus

One in five people who undergo rapid Covid testing could be incorrectly told they have the virus, according to new research suggesting accuracy rates may be lower than initially thought.

The study, published in the British Medical Journal (BMJ) on Thursday, examines the performance of finger prick antibody tests for SARS-CoV-2, the virus responsible for Covid-19 infection.

Results suggest that if 10 per cent of people given the test had previously been infected, around 20 per cent of all positive results would be incorrect, or “false positives”.

The findings are significant as they contradict those of previous, non-peer reviewed research, that found there was no inaccuracy among positive tests.

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Therefore, while they can deliver a sufficient degree of accuracy for surveillance studies of a population, laboratory confirmation of positive results would likely be required to provide evidence of any protection from the virus.

The research was carried out by scientists from Public Health England and the Universities of Bristol, Cambridge and Warwick.

They looked at the AbC-19 Rapid Test which uses a drop of blood from a finger-prick to assess how likely it is the person has previously been infected, delivering results in 20 minutes without the need for laboratory analysis. Such fast turnaround is invaluable in a world struggling to function in the ongoing pandemic.

In Ireland there appears to be no prospect of any rapid testing system being approved this year.

The research looked at blood samples in a laboratory from 2,847 key workers across healthcare, fire, and police from last June.

Of those, 268 had a previous positive result so were classified as “known positives”. The remaining 2,579 had unknown previous infection status. A further 1,995 pre-pandemic blood samples were also tested as “known negatives”.

They estimated that the AbC-19 test’s ability to correctly identify a true negative sample at 97.9 per cent, meaning 2.1 per cent of people who did not have a previous SARS-Cov-2 infection incorrectly tested positive.

They also estimated its ability to correctly identify a true positive sample at 92.5 per cent based on the confirmed cases but considerably lower (84.7 per cent) in those with unknown previous infection status.

This difference, the researchers explain, is probably due to the test being more sensitive when antibody levels are higher – those with a positive PCR result tended to have more severe disease, meaning they were likely to have produced more antibodies.

The lower figure of 84.7 per cent is thought to be a more realistic estimate level, meaning that 15.3 per cent of people with a previous SARS-CoV-2 infection would be missed by the test.

That means that if 1 million people were tested, of whom 10 per cent had been previously infected with SARS-CoV-2, there would be 18,900 false positive results. Overall, about one in five positive results would be wrong, they said.

Test accuracy could even be lower if used at home by members of the public, the research found.

In a linked editorial, Dipender Gill at Imperial College London and Mark Ponsford at Cardiff University, noted that "a clear message must be communicated to the public that positive results from these assays do not provide evidence of immunity".