The Reliability & Utility of Serological Antibody Tests in COVID-19

Of special note to rheumatologists, both patients with high anti-nuclear antibody levels & patients positive for rheumatoid factor show an increased rate of false positivity for these tests.

The FDA is actively collaborating with the National Institutes of Health’s National Cancer Institute, the Centers for Disease Control and Prevention and the Biomedical Advanced Research and Development Authority to independently validate some of these antibody tests. Information about test sensitivity, specificity and other factors from a limited number of such tests is now available (although these also do not include information from neutralizing PRNT studies).¹⁵

Utility of Antibody Testing in SARS-CoV-2

Much media attention has focused on the appearance of fraudulent tests and the validity of available tests, and deeper questions remain about when and whether such tests should be used. The issue is not just one of testing quality. Dr. Kadkhoda points out, “Usually when labs start offering a test, we know a lot about it, and we have enough evidence of its clinical usefulness. What happened in SARS-CoV-2 was that we did the reverse. A lot of labs started testing first, before we had that evidence.”

Dr. Kadkhoda adds, “There are some tests out there that are downright bad, and that’s why the FDA is now cracking down on them after its initial complacency. But even the best ones out there still fall short in terms of clinical usefulness—the science is just not there.”

The issue of false positivity is key, even for tests of the highest caliber. Ninety to 100 percent of adults have antibodies to common coronaviruses, a potential source of false positivity. Dr. Kadkhoda notes that other known factors can lead to false positives, such as flu and flu vaccination, syphilis, herpes virus, metapneumovirus and dengue. Of special note to rheumatologists, both patients with high anti-nuclear antibody levels and patients positive for rheumatoid factor show an increased rate of false positivity for these tests.⁷

Disease prevalence also influences reliability. Dr, Kadkhoda presents a scenario of <2% disease prevalence, a reasonable current estimate in much of the country. He explains, “Even with a sensitivity of 100% and a specificity of 99%, which would be unreasonably high, our positive predictive value would be very low. Up to half of results could be false positives. When the prevalence in an area is very low, the possibility of a false positive result is actually very high.”

Very Limited Diagnostic Role

The Infectious Diseases Society of America emphasizes these antibody tests should never be used as a single diagnostic tool. Dr. Calabrese points out that serological testing has never been routinely used for diagnosing respiratory viral infections like flu or respiratory syncytial virus, because it isn’t particularly useful. If performed early in the disease course, the test is very likely to be negative, because antibodies have not yet developed, particularly in more mild cases. PCR is a much more reliable option.

Dr. Kadkhoda also points out that most hospitalized patients test positive via PCR throughout the course of their hospitalization. He adds, “By the time IgM appears, the vast majority of these patients have already presented with things like septic shock or [acute respiratory distress syndrome], which is too late.”

Dr. Calabrese notes these tests may potentially play a very limited role in diagnosis in specific circumstances. “The duration of PCR positivity is variable and can turn negative a couple weeks into infection,” she explains. “If someone presents late into symptom onset, their PCR might be negative, and there might be a role for complementary antibody testing.” The high risk of false positivity should also be considered in such scenarios, particularly in an area of low disease prevalence in a person with no known disease exposure.

A potential exception is the new multisystem inflammatory syndrome in children (MIS-C) that is potentially linked to SARS-CoV-2 infection. Dr. Kadkhoda suggests that such serological antibody testing might be used in the context of an exclusionary diagnosis, if initial viral PCR were negative. But even here, he adds, a positive antibody marker wouldn’t necessarily establish diagnosis, and the test results would be unlikely to change medical management.

As Immune Status Marker

Some governments have suggested that antibody results might be used for “immunity passports” that might enable individuals to travel because they are presumed to be immune to re-infection. However, the World Health Organization and other health groups have strongly argued against this.¹⁶

Dr. Calabrese adds, “Some places are using these antibodies as an immune status check or even a back to work check. But that does not seem to be a good or safe idea. You could have positive IgG to COVID-19 but still be sick and shedding virus.” Patients testing positive for the virus—either as true positives or false positives—may stop taking recommended public health precautions.

Test specificity is an issue, but another problem is that the “correlate of protection” has not been established for COVID-19: We don’t fully understand the measurable signs that can be used to indicate a person is immune. “Compare it with something like hepatitis B virus,” says Dr. Kadkhoda. “We do serology for it; we know which antibody is the correlate of protection, and we know even what level of antibody equals protection.”

Complicating matters still further, at this point researchers cannot rule out the possibility of antibody-dependent immune enhancement, though thus far no data have directly suggested that this is the case.²

Through prospective vaccine trials, we may ultimately discover if antibody is the component of the immune system that might confer protection, and if so, what level might be needed. Potentially other factors, such as cellular immunity, might be important.⁷

Says Dr. Calabrese, “We have patients reaching out to us all the time asking for antibody tests so they can feel more comfortable going back out into society, but we strongly recommend against using a positive antibody test as a security blanket.” A role for serology may emerge once we know more about the natural history of the virus and the immune response.

Valid Uses: Serosurveys, Plasma Donor Screening, Vaccine Development

Serosurveys are a valid use of the antibody tests, and they provide a helpful avenue for epidemiological studies of disease prevalence. However, Dr. Kadkhoda points out that even for these purposes, health officials should view antibody test results with caution. He points out that in an area of low disease prevalence, even high-quality antibody tests will have a high rate of false positivity, giving an inflated number of individuals who have previously been infected. He says, “As a result, you may arrive at a low case fatality rate, and erroneously people may think this disease is not as dangerous as it is.”

Serology tests are also currently being used in convalescent plasma therapy. In this treatment, plasma containing antibodies from recovered patients is transfused to patients very ill with COVID-19. Serology tests are being used to identify candidates to donate plasma. Though some early, small studies have been promising, the most recent randomized controlled trial showed no clinical benefit.^17,18 Further controlled trials will be needed to demonstrate that the therapy is effective.

Tests for SARS-CoV-2 antibodies are also important for vaccine design and evaluation as part of clinical trials. This will require a test with high sensitivity—and even more importantly, high specificity—to assess the immune response after vaccination. Here, researchers would specifically need to identify neutralizing type antibodies.

Dr. Kadkhoda explains, “Immuno­genicity trials typically should include a neutralizing assay, not just an ELISA or other commercial assays. They might try those also, but it’s very important to have a neutralization assay to make sure those individuals make neutralizing antibodies against that spike protein of SARS-CoV-2.”