UK regulators have asked all coronavirus antibody test providers that use a blood sample taken with a finger prick to halt operations as doubts persist over their accuracy.
On Wednesday, the Medicines and Healthcare products Regulatory Agency’s director of devices, Graeme Tunbridge, said the use of unvalidated samples may lead to unreliable results.
“People who have purchased one of these sampling kits, and received an antibody test result, should not consider the result to be reliable and should not take any action on it,” he said.
Antibody testing will be pivotal in determining how far the new coronavirus has spread, and might give authorities and scientists an idea of whether, and how, immunity develops.
The assay enables healthcare professionals to determine whether someone has been exposed to the virus in the weeks before testing. Countries such as Germany have staked a lot on the initiative, hoping it will yield a fuller picture of how prevalent the virus has been.
The UK government has struck deals with diagnostics companies Roche and Abbott Laboratories to supply the NHS with antibody tests that will initially be reserved for healthcare workers. But those tests rely on blood samples drawn by a healthcare professional, and are not validated for use at home with blood samples taken from capillary vessels, like those used in finger prick tests.
“We are asking all providers of laboratory-based Covid-19 antibody testing services using capillary blood collected by a finger prick to temporarily stop providing this service,” Dr Tunbridge said.
Nevertheless, it is not illegal for vendors to use tests in a different way, which has not been endorsed by the manufacturer. This is known as “off label” use, and is what some private clinics that are selling Abbott’s test using the finger prick method are doing. Abbott has said it does not condone that type of use.
The MHRA’s decision should not be “confused with a recall or ostensible criticism of the labs, it is further due diligence,” said Eoghan Macsweeney, co-founder of CityDoc, a network of private general practitioners, which is offering the Abbott test.
He added that he was confident of the accuracy of the tests CityDoc had already done using the finger prick method, but added that clinics would soon also be offering the test using blood drawn from veins.
On Tuesday, the US Centers for Disease Control warned that current technology meant about half of all antibody tests done in areas with low prevalence rates would be considered inaccurate.
To be deemed accurate, antibody tests must be able to identify antibodies in blood samples that are specific to this particular coronavirus. No test is 100 per cent accurate, and all will generate some false results.
Depending on the prevalence of the virus in a population, “the number of false positives might outweigh the number of true positives,” said Joseph Fitchett, medical director at Mologic, a medical products manufacturer, which is developing Covid-19 antibody tests.
Deciding to whom antibody tests will be given is “something the government needs to work out,” he added.