What can COVID-19 antibody tests really tell us?

Antibody tests may offer a more complete picture of COVID-19 infection rates. Also known as serologic tests, antibody tests don’t diagnose active infection. Instead, they look for specific antibodies in the blood—protective proteins forged in response to a SARS-CoV-2 infection.

Serologic tests should not replace direct detection methods, such as polymerase chain reaction (PCR) or antigen testing. They do, however, have several important applications in monitoring and responding to the COVID-19 pandemic.

These tests can help determine the proportion of a population previously infected with SARS-CoV-2 and provide information about populations that may be immune and potentially protected from reinfection. Thus, demographic and geographic patterns of serologic test results can help determine which communities may have experienced a higher infection rate.

“Serologic testing can help us understand how many people have been infected and recovered,” says Dr. David Boyle, PATH’s chief scientific officer in the Diagnostics Program. “If you only count those people who presented for testing and had a positive PCR result, then you’re only seeing the tip of the iceberg.”

Antibodies against SARS-CoV-2 infection may confer some degree of protection against the novel coronavirus. But scientists cannot yet confidently confirm how strong the protection may be or how long it may last. Adding to the uncertainty—some early point-of-care antibody tests delivered a high number of false positives, mistakenly signaling that people had been infected when in fact they had not been infected.

Because there are still many unknowns, the U.S. Centers for Disease Control and Prevention cautions against using antibody tests to guide important decisions about when people can return to work, school, church, or other gathering spots.

That’s disappointing news for those who hoped the antibody tests might point the way to safe reopening.

So what makes antibody tests susceptible to delivering false positive results? Here are three factors, and potential countermeasures for each:

1. Methodology

In April, a serologic survey in California’s Santa Clara County suggested that the local rate of SARS-CoV-2 infection was up to 85 times higher than the number of laboratory-confirmed cases at the time. But scientists quickly pointed out several flaws in the study’s design.

Among them: poor test quality (so far, existing and newer antibody tests are still being evaluated for performance); consent bias (it is likely that many participants signed up because they’d been sick and were seeking confirmation of a suspected COVID-19 infection); self-selection (participants volunteered for the study via Facebook ad); and lack of ethnic diversity (younger white women predominated).

“Serologic studies are essential to understanding the reach and impact of COVID-19,” says Roger Peck, senior program officer for PATH Diagnostics, “but they must be guided by rigorous protocols. We need controls, verified patient histories, and—of course— confidence in the accuracy of the tests.”

2. Inconsistent test validation

In the United States, the Food and Drug Administration (FDA) initially allowed manufacturers to bypass its usual lengthy vetting and validation process. Test makers could self-validate the quality of their products, as long as they included a disclaimer with the results.

The agency has since revised this policy, and now requires that test developers apply for an Emergency Use Authorization—still a less rigorous protocol than the standard approval process.

To support test developers in validating their diagnostic tests, PATH offers the Washington COVID-19 Biorepository. This library of clinical samples includes qualified, pedigreed sets of nasal swabs, tongue swabs, serum, and plasma. With access to these resources, free of charge, developers can assess and improve the performance of their assays before taking them to market.

3. Statistics

By September 2020, just over two percent of the total U.S. population had confirmed infections of SARS-CoV-2.

When a serological study tests a large number of people in an area with low prevalence of COVID-19, the false positives add up quickly and may skew the results. This is due not only to the individual test performance, but also a reflection of the prevalence of COVID-19 within the population.

“As the possibility of infection becomes lower,” says Allison Golden, scientific program officer for PATH Diagnostics, “there is a greater chance of encountering a false positive result than a true positive result.”

One potential remedy for false positives is repeat testing to confirm a positive result. Conducting a confirmatory test has become routine for some infections, including HIV/AIDS.

Although antibody tests are not used to diagnose acute infections, they are crucial to understanding the broader scope, reach, and impact of the COVID-19 pandemic. As we conduct larger population-based studies, the consequences of individual false positive results will become less severe.

In Indiana, a serologic study of 4,600 people indicated that COVID-19 was both more prevalent than previously believed, and less deadly. Large-scale studies are also under way in the U.S. and elsewhere through the National Institutes of Health, the World Health Organization, and the German Disease Control and Prevention Agency.

“Well-designed studies with proven serologic tests will give public health officials a clearer picture of the prevalence and impact of COVID-19,” says Paul Drain, associate professor in the Division of Allergy & Infectious Diseases, University of Washington. “However, they won’t tell us when it’s safe for people to return to school or work—or any public place—so we still need to pursue better and faster PCR and antigen diagnostics for acute infection.”

Learn more about PATH’s work in diagnostics.

*Photo: Ministerio de Defensa del Perú