SARS-CoV-2 serology tests don’t necessarily live up to their hype as a diagnostic and screening tool, caution two experts in Clinical Chemistry. Christopher W. Farnsworth, PhD, and Neil W. Anderson, MD, D(ABMM), of the Washington University School of Medicine Department of Pathology in St. Louis, advise that clinical labs do a thorough vetting of these tests to assess their suitability.

Patients and providers alike should exercise caution when ordering and interpreting results from antibody tests, Anderson told CLN Stat. “False negatives can occur early in the disease process, and false positives can occur secondary to cross-reacting antibodies to other pathogens (such as seasonal coronaviruses). In some instances, when testing low-risk asymptomatic patients, you may be more likely to get a false positive than a true positive. In these instances, testing can become misleading and harmful.”

Serology tests have been touted as a way to identify COVID-19 infections, support community surveillance efforts, and identify convalescent plasma donors. Farnsworth and Anderson issue a reality check on all three counts. At the time of this article’s drafting, “we wanted to bring to light the limited data regarding most of the assays on the market … most of which were released without an [emergency use authorization] EUA,” Farnsworth told CLN Stat.

An unprecedented number of tests have appeared on the market since the beginning of the pandemic. The U.S. Food and Drug Administration (FDA) has tried to maintain quality control through several actions. On May 4, it updated its EUA policies for COVID-19 antibody tests, requiring developers to submit their requests and validation data within 10 business days. The agency also set performance characteristics for approved tests, including 90% sensitivity and 95% specificity and added a new section to delineate which tests should no longer be distributed.

The authors agreed with FDA’s decision to require that test developers receive an EUA to offer serologic testing. However, “our opinion on the utility of antibody testing for SARS-CoV-2 has not changed,” since their article was first published in April, said Farnsworth. Consistent with guidance from the Infectious Diseases Society of America (IDSA), the clinical value of antibody testing has not been fully demonstrated and continues to show limitations, he noted.

Using serology tests to identify infected patients, for instance, needs further study. “It cannot, at this time, determine whether an individual is immune or not. Also, when used in patients with low pretest probability, even assays with apparently good specificity can have very high rates of false positives and poor positive predictive values,” said Anderson.

There’s also a presumption that recovered patients with high anti-SARS-CoV-2 titers would serve as ideal donors. Many serology tests, however, provide qualitative rather than quantitative results, making it difficult to distinguish between donors with high or low antibody titers. “If there is an intent to convert qualitative assays to quantitative assays for this purpose they must be evaluated as such (including an assessment of precision spanning the reportable range),” suggested Anderson and Farnsworth.

Compounding these strategies is the fact that little is known at this time about protective, neutralizing effects of SARS-CoV-2 antibodies. This makes it difficult to track the virus within a community. Low seroprevalence in reportedly high-prevalence SARS-CoV-2 areas offers another challenge to successful population surveillance with antibody tests, they continued.

“Even a laboratory test with excellent sensitivity and specificity is often unhelpful in low-prevalence settings,” said Anderson and Farnsworth.

Since Anderson and Farnsworth’s article was published, several manufacturers have released EUA assays with large cohorts of pre-pandemic specimens to assess specificity. Several independent studies have verified such assays, including two co-authored by Farnsworth and Anderson that examined the Roche and Abbott SARS-CoV-2 tests. While the findings support the use of these assays for seroprevalence studies, “there is still a high likelihood for false positive results in areas with low prevalence,” said Farnsworth.

Clinical labs can offer solutions through rigorous validation studies of these serologic assays, recommended Anderson and Farnsworth. The Clinical and Laboratory Standards Institute’s recommendations on sensitivity and specificity offer some guidance. However, “the rigor of the validation should ultimately be dependent on the intended test population,” stressed the authors. To achieve a high positive predictive value in populations with low seroprevalence, for example, a test should meet >99% with small confidence intervals.

Anderson and Farnsworth also suggested that:

  • Clinical labs analyze several sources of specimens to assess a test’s false positive rate. This includes symptomatic patients who tested negative for SARS-CoV-2 by molecular assays and specimens acquired before the COVID-19 pandemic began.
  • Each validation/verification includes a subset of patients with confirmed seasonal coronaviruses such as NL63, HKU1, OC43, and 229E.
  • Testing take place in specimens with potential cross-reactivity, such as those with heterophile antibodies and rheumatoid factor.

The U.S. Centers for Disease Control and Prevention (CDC) has suggested the use of a second test to confirm results in an orthogonal approach in low-prevalence populations. “However, very few labs have adopted this approach,” said Farnsworth.

As Anderson and Farnsworth call for more data to support the clinical utility of serology tests, another perspective cautioned against questing for unattainable ideals. “There is no such thing as a 100% safe bet” with antibody tests, wrote Milton C. Weinstein, PhD, Kenneth A. Freedberg, MD, Emily P. Hyle, MD, and A. David Paltiel, PhD, in The New England Journal of Medicine (NEJM).

The World Health Organization’s (WHO) assessment that antibodies or antibody tests don’t guarantee immunity is correct, wrote Weinstein and colleagues. “But we believe that the WHO is dead wrong to suggest that we cannot act until we ‘guarantee’ the accuracy of the immunity-certification process,” they asserted. A pandemic of this magnitude deserves action, and doing nothing while waiting for the science to be perfect would have serious health consequences, they contended. “The bottom line is this: We have enough evidence and expert opinion to make an informed decision today. And we can put the monitoring systems in place to learn from that decision so that we can make even better choices tomorrow,” they said.

Farnsworth said he couldn’t directly compare the NEJM article with his own analysis. “The authors are arguing for public policy, whereas we are trained in laboratory medicine. It is certainly against the grain of what the WHO, CDC, and IDSA are recommending regarding the need for more data. Seroprevalence studies are emerging, showing the infection rate is higher than PCR testing has indicated. However, the prevalence is still largely unknown (CDC cites <5%-25% across the country) and is likely lower than 1% in rural areas,” he said.

Serology testing is just one piece of evidence that should be carefully weighed against others, noted Anderson. “In this way we can acknowledge both its utility and its limitations,” he said.