The age of molecular testing for the COVID-19 pandemic is still with us, but the emphasis in the months ahead will be on serological testing as a quicker, more useful mass testing alternative. However, test developers have a number of hurdles to overcome in devising these serological tests, including that antibodies for the virus’s antigens emerge at different times in the disease cycle, just one of several challenges that have to be met in the effort to bring the SARS-CoV-2 virus to heel.

The FDA has granted emergency use authorizations to a number of tests, but nearly all of those have been tests designed to detect viral RNA via polymerase chain reaction methods. Test developers and researchers are hard at work coming up with new approaches to testing, such as research into loop-mediated isothermal amplification. This approach, which the authors claim offers sensitivity comparable to PCR, can be deployed as a point-of-care test, which is not always the case with PCR.

One of the problems with serological testing as that antibodies for the antigens presented by the SARS-CoV-2 virus show up at different stages of the disease cycle. There are also questions about whether a positive serological test suggests immunity for the patient, and these and other questions must be answered before serological testing can begin playing a role in restoring the world’s economies and thus stave off a global economic disaster.

‘Use what you have’ the clinical lab mantra

David Grenache, president-elect of the American Association for Clinical Chemistry, told BioWorld that a clinical lab’s choice of testing often hinges on the types of equipment that lab has installed. Grenache, who is also the chief scientific officer for Tricore Reference Laboratories Inc., of Albuquerque, N.M., said the question of which antibodies confer immunity is one of several million-dollar questions. He noted that the literature, including some that has not been subject to peer-review, includes some suggestions that antibodies “that are directed against the viral spike protein are likely the ones that confer some protection.” Still, Grenache cautioned that this and other questions are still in the discovery phase.

The first class of antibodies produced is immunoglobulin M (IgM), an oft-cited early serological marker of infection, and Grenache said classic immunology holds that immunoglobulin G (IgG) is detectable later. IgG in other viral infections may be detectable for a long period of time, months to years, in some instances, but he said it is too early to say how long IgG will persist with SARS-CoV-2. IgG is typically the type of marker that correlates well with sustained immunity to reinfection.

“You can look to what has happened in the past with other coronaviruses as a predictor of what could happen now,” Grenache said, adding that other species of coronavirus usually hit the respiratory system fairly mildly. Conversely, the frequency with which humans experience the common cold suggests a failure to generate a long-term immune response, although viral mutation might explain this as well. “You could predict that you might have short-term immunity that might wane,” he said, adding, “we can speculate, but we can’t be 100% certain” whether any of these scenarios describes the ultimate trajectory of COVID-19.

One of the concerns about the serological testing field is whether there is some cross-reactivity for a test to other viruses, with other versions of the coronavirus the likely targets of cross-reactivity. A test can be designed to pick up a single antibody, but a test for multiple antibodies is more useful for maximizing detection, particularly given the habit of some antibodies to come and go over the course of the disease. Grenache said some of the early data suggest that the lag time between infection and production of antibodies is only several days, but that won’t always be the case, particularly for those in their elderly years and those who are immunocompromised.

“There’s this growing misunderstanding that a serological test is a substitute for a molecular test,” Grenache said, but he cautioned, “you can still be in the early stages of the infection and shedding virus, yet have no detectable antibodies.” Hence, PCR testing for viral RNA will still be important in the weeks and months ahead.

While seroprevalence testing is topical among researchers, physicians are still dealing with the individual patient. Depending on how the patient presents, some physicians may just assume COVID-19 and treat based on that assumption, but Grenache said policymakers will need more information about what serological testing offers before it can be used to determine who can return to work, regardless of whether a specific treatment has been identified. The identification of a neutralizing antibody would also help, but uncertainty will rule the day until one of these factors changes significantly.

Grenache said he is confident that there is more than one serological test that avoids the cross-reactivity problem, but that he is concerned that there are tests out there that offer questionable quality as well. A lot of these tests being marketed to doctors, and Grenache said, “that’s what gives me great concern, the number of tests that have flooded the market without any understanding of their performance characteristics.” The use of such a test is “egregious and dangerous,” he said.

IgM deemed more likely to exhibit cross-reactivity

Christi Wojewoda, vice-chair of the College of American Pathologists microbiology committee, said the available data on serological testing are “very interesting on a population level, but we don’t have enough information to use it yet on an individual patient.” This is because of the uncertainty surrounding the implications of a positive test for antibodies in terms of immunity, and thus doctors will be reluctant to offer their patients any advice about returning to work until more data emerge.

Another question is whether a serological test will ever be able to disclose whether a patient is shedding the virus, which often requires a cultured specimen from the patient, but Wojewoda pointed out that IgM antibodies have a reputation for cross-reactivity. Consequently, a serological test for IgM only has to be confirmed via PCR testing, whereas a test that is positive for IgG is more reliably specific for the SARS-CoV-2 virus, all of which resurrects the question of whether a single antibody serology test is ideally suited to the current predicament.

Wojewoda said that because antibodies are not confirmatory for immunity, “we need to learn more about potential re-exposures” as this first wave of COVID-19 unwinds. There are other elements in the immune system that may prove helpful, but she said, “I think the first piece of data that would be relatively easy to obtain is what is our overall percent of patients who have been infected or exposed.” To date, this denominator question has been difficult to address because of the low volume of PCR testing, but she said, “just to get our arms around our true denominator would be helpful,” at least in terms of establishing the risks with the second wave. This would help to answer the question of whether patients that showed antibodies on the first wave will get sick again. “That’s going to take some time,” Wojewoda said, but “that’s what we’re all looking for.”

“The thing that concerns me most about serological testing is having people falsely assume they are immune and can go out into the community without any precautions,” Wojewoda said, particularly given that the social distancing and the associated precautions seem to be working as intended. She noted that there are data emerging that have begun filling in the picture for the role of serological testing, but those data “are coming in dribs and drabs.” Those data points will eventually fill in the picture in its entirety, but as to when the entire picture will emerge, “it’s hard to put a time frame on it,” she said.