New antibody tests are not 100% accurate on all measures
4th Jun 2020
New antibody tests are 100% accurate.
This is not correct. Although Public Health England found the Roche test had 100% specificity, it did not have 100% sensitivity.
“One hundred per cent accuracy of test developed by Swiss firm Roche confirmed by experts at PHE's Porton Down facility last week.”
The Telegraph, 13 May 2020
“A new test to determine whether people have ever been infected with coronavirus is 100% accurate, public health leaders have said.”
ITV News, 14 May 2020
“Swiss pharmaceutical giant Roche's test is 100 per cent accurate, meaning it will identify everyone who has had COVID-19.”
Mail Online, 14 May 2020
It was widely reported in May that a new antibody test—which can detect if someone has had Covid-19—is ‘100% accurate’. However, this is not quite correct. Although the Roche test was found to be completely accurate when it came to identifying samples from someone who had not had Covid-19, it missed 16% of samples of people who had.
The accuracy of the antibody tests is based on two different measures: the specificity—the percentage of the samples that are negative and are accurately identified as negative by the test—and the sensitivity, the percentage of the samples that are positive and are accurately identified as so. As such, although it is correct to say the Roche test has 100% accuracy in specificity, it is not 100% accurate in sensitivity.
The claims in the media about the 100% accuracy of the Roche test was based on an evaluation of the test undertaken by Public Health England (PHE), although the evaluation reports themselves were not published until a week later. This evaluation showed that the Roche test had a specificity of 100%, but a sensitivity of 83.9%—increasing to 87% if the sample was taken 14 days or more since a person had developed symptoms. This means that, of the 93 samples taken—a relatively small sample size—from people who did have Covid-19, the test incorrectly said 15 were negative.
In trials done by the tests’ own manufacturers, the Roche test was reported to have a specificity of 99.8% and sensitivity of 100% after 14 days. The Abbott test was found to have a specificity of 99.6% and sensitivity of 100% after 14 days.
Alongside the evaluations, PHE released a comment from Professor John Newton, national coordinator of the UK Coronavirus Testing Programme.
Prof Newton said: “Last week, scientific experts at PHE Porton Down carried out an independent evaluation of the new Roche SARS-CoV-2 serology assay in record time, concluding that it is a highly specific assay with specificity of 100%. This is a very positive development because such a highly specific antibody test is a very reliable marker of past infection. This in turn may indicate some immunity to future infection although the extent to which the presence of anti-bodies indicates immunity remains unclear.”
Professor Jon Deeks, Professor of Biostatistics and head of the Test Evaluation Research Group at the University of Birmingham, told the Science Media Centre that the reports from PHE made it “clear” that saying either test was 100% accurate was “misleading”.
“For the benefit of the public it is important that the performance of these tests is correctly reported and disseminated, and that the public are given factual information to correctly choose whether, how and when to use these tests, and the certainty with which their results should be interpreted.”
Prof Deeks warned about “notable limitations” in the evaluations, including a lack of information on the origin and severity of the disease in the samples and the fact that non-Covid-19 patients with similar respiratory illnesses were not included. He also noted there was a margin of error on the figures due to the small sample size used.
He also warned that some websites are now offering home sampling kits which make “false claims about the performance of these tests” and fail to mention that their accuracy increases depending on the time since the onset of symptoms. He said some of these websites “repeat the misleading statements made in the media that these tests have been approved by PHE”. PHE evaluates but does not approve tests.
Full Fact put Prof Deeks’ concerns to PHE. A spokesperson said: “These are new tests in a rapidly evolving field of work. Our evaluations have been completed in record time using the samples and tests that were available to us. We are confident that the volume of samples and methodology was of a high standard.
“The purpose of our evaluation is to inform decisions and recommendations within the government’s testing strategy. However, any laboratory using these tests is still required to complete their own evaluation to ensure the tests perform as described—our work is designed to reduce the amount of local work required. Our evaluation work is ongoing and as more tests and sufficient samples become available, we will continue to refine our approach.”