Antibody tests at 98% accuracy would lead to 27% of immunity diagnoses being incorrect, government advisers warn

AFP via Getty Images
Luke O'Reilly5 May 2020

Antibody tests at 98 per cent accuracy would put up to a quarter of population at risk of infection, government advisers have warned.

The Medicines and Healthcare products Regulatory Agency has set a minimum standard of 98 per cent accuracy for antibody test kits.

However, documents published on Tuesday reveal that tests with 98 per cent accuracy would still leave a quarter of those told they were immune at risk of infection.

The documents, which have been seen by the Scientific Advisory Group for Emergencies (Sage), say that if five per cent of the population are thought to have antibodies, then from a random sample of 1,000 people this will equate to 50 people who have antibodies.

At 98 per cent accuracy, only one of those 50 people will be said to not have antibodies when they actually do.

But according to the documents, the real problem arises from those who do not have antibodies being told that they do have antibodies.

Even 99% accuracy would leave 16% of those told they had antibodies exposed to infection 
POOL/AFP via Getty Images

If five per cent of the population have antibodies then this leaves 950 people out of 1,000 who do not have antibodies.

At 98 per cent accuracy, 931 of those 950 people will be correctly identified as not having antibodies. This leaves the remaining 19 as being incorrectly identified as having antibodies.

So 49 people who have antibodies will be told they have them and 19 people who not have antibodies will be told they have them. This is a total of 68 people being told they have antibodies.

If 19 of those 68 people are being told they have antibodies, but do not have antibodies, then that means that 27.9 per cent of those 68 people are being told they have antibodies - when they do not have antibodies.

The document's explosive conclusion is that even with 98 per cent testing accuracy, 27.9 per cent of people told they have antibodies would not have antibodies.

Even if an antibody test is 99 per cent accurate, under the same reasoning it would still incorrectly tell 16 per cent of people that they have antibodies when they do not.

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It comes as Quotient, a Swiss company with scientists based in Edinburgh, has developed a new antibody test that they has 99.8 accuracy.

Quotient have warned that the UK could miss out on the test due to interest in Europe.

Meanwhile, Sir Patrick Vallance told the Health and Social Care Committee on Tuesday that evidence from around the world suggests the vast majority of people who have had the infection have “some form of antibody response”, adding “so that looks quite promising”.

He said: “What we don’t know is the degree of immune protection that provides.”

He said “some degree of protection” would be expected, but that it “almost certainly” will not provide absolute immunity.

Whether someone can still carry the virus and be infectious after having developed antibodies also remains unknown, he said.

Giving a potential timeframe for immunity based on experience from other coronaviruses, he said: “It may last for one, two, three years, but not for many, many years.”

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