Caution urged over reliance on Covid-19 antibody testing

Employers hoping to get more staff back to work by using coronavirus antibody tests have been warned that they could miss one in 10 cases of Covid-19.

According to researchers at the University of Birmingham, the tests are not accurate enough to confirm whether someone has had the virus if used in the first two weeks after symptoms appear.

They said antibody tests are only likely to be useful in detecting previous Covid-19 infections if used at least 14 days after the onset of symptoms.

Jon Deeks, professor of Biostatistics and head of the Test Evaluation Research Group at the University of Birmingham, said: “We’ve analysed all available data from around the world and discovered clear patterns telling is that timing is vital in using these tests. Use them at the wrong time and they don’t work.

“There are promising tests, but there isn’t enough evidence to be able to identify the best tests yet, and we need to continue analysing data as it becomes available – particularly from non-clinical settings where individuals are displaying less severe symptoms, and following people over longer time periods.”

The researchers cautioned against over-reliance on antibody testing, as the studies into their effectiveness have been small-scale and involved selected hospital patient groups. They analysed data from 54 studies published before the end of April 2020, covering around 16,000 test results.

Although the body produces three classes of antibody – IgA, IgG and IgM – most antibody tests only detect IgG and IgM. The IgG antibody is usually more prevalent a few weeks after infection.

The studies showed that the combination of IgG or IgM had a sensitivity of 72% for eight to 14 days; 91% for 15 to 21 days; and 96% for 21 to 35 days. Little data were available beyond 35 days.

Where around half of employees may have had Covid-19 – such as in health care settings – the researchers estimated that there will be seven false positives and 43 false negatives for every thousand people undergoing combined IgG and IgM testing 15 to 21 days after Covid-19 symptoms appeared.

Deeks said: “In time, we will learn whether having previously had the infection provides individuals with immunity to future infection, which will indicate the personal value of having this test.

“This is a fast-moving field and we plan to update this review regularly as more studies are published. However, the design, execution and reporting of studies of the accuracy of Covid-19 tests requires considerable improvement, and action needs to be taken to ensure that all results of test evaluations are available in the public domain.”

The study was published as 14 senior academics warned antibody tests are being rolled out in the NHS and care settings without adequate assessment.

In an open letter published by the BMJ, the scientists warn the tests do not prove immunity and have “no benefit” to healthcare settings as they do not change what personal protective equipment staff must wear, for example.

But Cathal Friel, chief executive of antibody testing kit manufacturer hVIVO, said hesitating to test staff “may prevent valuable, safe treatments from seeing the light of day and making the UK workforce productive once again”.

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“While there is no 100% conclusive evidence on antibodies completely avoiding reinfection or delivering long-term immunity, the issues around antibody testing accuracy and compliance with EU standards have largely been addressed already,” said Friel.

“Our tests are safe and available now; waiting for the perfect solution will deprive employers of an important tool in ensuring the UK’s economic recovery and so carries serious economic risks.”