Testing for the coronavirus has been a hot button both for political reasons and for medical reasons. There are actually two different types of tests. One tests for the presence of the active virus, or do you have it and the other for the presence of antibodies in the blood which would indicate that you had the virus.
There are currently dozens of antibody tests available for the coronavirus. The test’s popularity is directly tied to the hope that it might offer a strategic vehicle that would allow us to open up the economy in a safe fashion. The thought process is that if these tests tell us which people have developed antibodies, thus immunity to the virus, these individuals could let us know who could safely return to work. However, according to the experts the reality is more complicated and depends on how results are used.
It is hoped that antibody tests could help us understand the extent of the spread of the virus as well as who might be immune, however, the tests are less informative than we had hoped about aa person’s past exposure or protection against future infection.
According to Tara Smith, a professor of epidemiology at the Kent State University College of Public Health, “The focus right now is primarily epidemiological.” An epidemiological approach means that we are trying to figure out the percentage of the population that has already been infected, even if some individuals never showed symptoms. “This will allow us to better calculate the fatality rate and to determine how far we still have to go to reach [infection] levels that would place us in the range of herd immunity,” or when a large proportion of a population has become immune to a disease because of vaccination or past infection. “It will also allow us to start looking at duration of immunity.”
Blood tests have already been conducted in communities across the U.S., and their findings vary widely. Estimates of positive antibody prevalence range from almost 25 percent in New York City and 32 percent in Chelsea, Mass., to between 2.8 and 5.6 percent in Los Angeles Countyand 2.8 percent in Santa Clara County in California.
These results support what experts already suspected based on case studies of asymptomatic transmission: COVID-19 is much more widespread than hospital data would suggest. However, several of the studies have been criticized by scientists, who have raised red flags about sampling methods, potentially flawed statistics and results that are first announced as press releases rather than as peer-reviewed or even preprint studies.
These problems and the lack of transparency are worsened by subpar assays. Many of the tests currently flooding the market have not been validated or verified by third parties, an important usual step prior to the use of these types of tests. Many experts have also said that even those that have received emergency use authorization from the U.S. Food and Drug Administration might not be accurate enough to assess disease prevalence levels outside of hotspots.
The Johns Hopkins Center for Health Security maintains and regularly updates a Web site that lists key characteristics of many of the blood tests for SARS-CoV-2, the virus that causes COVID-19, on the market and in development.
Experts recommend that before they are made available and given that such important decisions are being made based on these tests, they need to be validated. At a minimum, experts recommend that validation studies include at least 100 positive and negative patients whose infection status is confirmed against a reference standard such as diagnostic test results and symptoms. In many cases these validation studies have not been made.
Antibody tests currently on the market have been validated in samples ranging from only a few dozen individuals to more than 1,000. Currently, the Center for Health Security lists tests approved for research or individual use in the U.S. that accurately detect antibodies in people who have them—a statistic known as sensitivity—between 82 and 100 percent of the time. Their ability to correctly identify antibodies only in those who actually have them—known as specificity—ranges from 91 to 100 percent.
On the surface, those numbers seem pretty good. But, according to Sarah Cobey, an associate professor of ecology and evolution at the University of Chicago, “threshold is set by context. So, if the seroprevalence, or the proportion of the community that has antibodies against SARS-CoV-2, is 3 percent versus 5 percent, you have to have an exceedingly good test” to distinguish that. If you’re [only] trying to identify if the prevalence is above 50 percent or below 50 percent, you can get away with a test that’s maybe less good. But nobody is in that category [with COVID-19].”
This variability in what constitutes an acceptable test arises from the fact that in populations with a higher prevalence of a disease or past exposure to it, true positives (individuals who test positive and have antibodies to the illness from a prior infection) and false negatives (those who test negative but actually have antibodies) are more common. Meanwhile in populations with a lower prevalence, tests are more likely to give false positives.
The bottom line is that we need to be very cautious before we have one of these tests and rely on both their accuracy and how we interpret the results. Are we really measuring what we say, is there really a presence of antibodies? Also, what is the actual meaning of the presence of antibodies? Does having antibodies mean that we are immune? What level of antibodies might be required to provide immunity and how long might any immunity last?
These questions are so important and we do not yet have the answers. So, don’t just go out and have an antibody test and assume that if you are positive you are immune. Even with a positive test, you must continue to follow all the safety rules, including social distancing, wearing a mask outside of your home and limiting social contacts.
