LONDON – Getting a handle on the specificity and sensitivity of rapid COVID-19 antibody tests, how they compare to each other and how they should be applied in population screening to understand who has had the virus and how it has spread, has been a source of difficulty and dispute between manufacturers, clinicians and public health experts.

One thing is certain, SARS-CoV-2 antibody detection tests have limited usefulness for early COVID-19 diagnosis since it can take 10 days or more after onset of symptoms for patients to become positive for detectable antibodies.

Following a scathing review of tests on the market published by the European Commission in April 2020, the EU Medical Device Coordination Group (MDCG) has now set out minimum requirements on how rapid antibody tests should be validated, how accurate they must be, and how they should be used.

COVID-19 rapid antibody tests should reach at least 98% specificity, the guidelines say. There is more leeway on sensitivity, with MDCG referencing the World Health Organization’s 90% requirement, rather than setting its own mark.

After complaints that manufacturers were not providing adequate evidence of how tests were validated, MDCG says the technical documentation for each test must contain data proving the claimed performance from studies conducted in the target population.

The positive sample panel should include at least 200 samples from people with a confirmed diagnosis of SARS-CoV-2, with details on timing between symptoms and sampling. Given that sensitivity of any test hangs on the time between contracting the virus and sample taking, manufacturers are advised samples should be from various stages and severity of disease.

If it is not possible to get access to well-defined panels reflecting antibody responses at different time points after infection, a rapid test could be calibrated against results from an established device that already has been shown to meet the requirements.

Similarly, the diagnostic specificity evaluation should include at least 200 negative samples from people tested for SARS-CoV-2 and confirmed as negative, or samples collected prior to November 2019.

Depending on the target population, these samples should represent age, gender, demographics and factors such as previous infectious disease history.

MDCG got to work on the guidelines after the review of the performance of COVID-19 test methods published by the European Commission in April 2020 exposed a number of gaps. The performance review concluded, “A comparison of the available antibody tests is not possible, because there is an almost complete lack of proper validation and standardization among antibody targeting methods.”

The reported performance parameters were difficult to compare, the review said. For example, it was not always stated how long after infection the samples were taken, and the threshold level of antibodies required to trigger a positive result varied between tests.

The commission’s review also complained that information on performance parameters was generally self-reported by the manufacturer or distributor of the device, with no access to details or raw data for the studies on which they were based.

In an analysis of close to 100 immunoassay devices, data on false negatives, false positives and cross-reactivity were largely unreported.

In the first wave of the pandemic, when PCR testing for the virus was being scaled up, and when lateral flow antigen tests for directly detecting the presence of SARS-CoV-2 were in short supply, rapid antibody tests were promoted as a faster, cheaper way of tracking infection.

Although CE marked antibody tests were available, some cautioned against their use, citing studies suggesting that the majority of patients develop antibody responses only in the second week after onset of symptoms, meaning a diagnosis of COVID-19 infection would be rather after the event, when opportunities for clinical intervention, or to limit transmission of the infection, had expired.

The World Health Organization advised in April 2020 of potentially negative impacts on public health if people who received a positive result assumed they were immune and dropped social distancing behaviors, increasing the risk of continued transmission. According to WHO’s scientific position paper, the use of rapid antibody tests in the response to COVID-19 had not then been established; there was conflicting evidence on the use and efficacy of the tests; and the public health consequences potentially outweighed any benefit an individual might get from this type of test.

Defining the state of the art

As the starting point for drawing up the new guidelines, MDCG looked to the EU directive on in vitro diagnostics, which says manufacturers must justify why a device is suitable for the intended purpose claimed on the label “in light of the state of the art.”

MDCG reviewed the instructions for use of 102 COVID-19 rapid antibody tests that had been placed on the market in the EU before September 2020, to assess the state of the art from the perspective of commercially available products.

As MDCG acknowledged, this “does not necessarily imply the most technologically advanced solution[s], but “what is achievable by a majority of devices” and which de facto sets the standard for new products.

Certainly, technology for rapid detection of anti SARS-CoV-2 antibodies has moved on in the year since the European Commission’s unfavorable review, and can be expected to further improve. For example, the first tests are beginning to come to market which can show if people have developed antibodies specifically in response to being vaccinated against COVID-19. These are designed to detect IgG antibodies to the SARS-CoV-2 full trimeric spike protein, as opposed to detecting IgM and IgG antibodies to its nucleocapsid protein.

In light of this progress, MDCG says its guidance, “may be seen as the minimum expected for devices being placed on the market at the time of publication.” Manufacturers are expected to continuously track technical advances, update performance data and adjust the intended purpose. If this is not possible, it is necessary to reassess if a device remains in conformity, MCDG says.

Defining the state of the art in qualitative or semi-quantitative COVID-19 rapid antibody tests is one thing, defining the intended use is another. The strength of antibody response depends on factors including age, severity of disease, the length of time between infection, use of certain immune-modulating drugs, or having chronic infections, like HIV, that suppress the immune system.

The guidelines say manufacturers should provide “complete and precise” information on aspects including the target population; the window between infection and antibody detection; how results should be interpreted; the target viral antigens and the types of antibody that are detected; if the test is designed to track antibody response during recovery, to detect previous infection, or to assess response to vaccination.

And it must be made clear that rapid antibody tests should not be used for first line diagnosis.

The instructions for use must set out the performance of the device in relation to all the intended use parameters, the guidance says.