The COVID-19 antibody test checks your blood for an immune system response to the virus SAR-CoV-2, the cause of COVID-19 illness. COVID-19 antibodies can be found even in people who have been exposed or infected but never had symptoms or have already recovered from the disease. Recent studies have indicated that more than half of the cases of people with SARS-CoV-2 infection may have mild or no symptoms. Serologic antibody tests not only can confirm whether a recent illness was COVID-19, it can also reveal who was infected and didn’t know it.
Unlike PCR testing, the nasal or throat swab test that identifies the presence of the genetic material of the SARS-CoV-2 virus, COVID-19 antibody testing should not be used to diagnose an active or current infection. It is intended to identify those who have been previously exposed or were infected in the recent past.
COVID-19 antibody tests look for two classes of antibodies, IgM and IgG. These antibodies typically start to show up in your blood about 1-2 weeks after the start of the infection. IgM antibodies are usually the first class of antibody to develop in response to infection. However, some people may not develop a high level of IgM or even have an IgM response at all. IgG antibodies generally appear later.
Experience with other viruses suggests that individuals whose blood contains antibodies associated with SARS-CoV-2 infection—provided they are recovered and not currently infected with the virus—may have some degree of immunity against reinfection. There is still uncertainty in the medical research as to how effective and long-lasting such immunity might be. As we learn more, it is possible that individuals with adequate levels of COVID-19 antibodies may be able to resume work and other daily activities in society. They may also be eligible to serve as potential donors of convalescent plasma that provides antibodies to help treat others with current infections.
At Princeton Longevity Center we start with a simple screening test done that can be done either with a drop of blood from a fingerstick or we can use the blood that we draw for the routine blood tests as part of your Comprehensive Exam. Using a type of test known as lateral flow assay, the results are available within about 15 minutes. This type of test has not been reviewed by the FDA and should not be used as the sole basis to diagnose or exclude SARS-CoV-2 infection or to inform patients of their infection status. This type of test provides a simple positive or negative result for both IgG and IgM antibodies. It does not provide quantitative information.
Because of the potential for any test to cause a false-positive result, we recommend that all positive tests include a confirmatory test at a commercial laboratory.
Scientists assess the accuracy of diagnostic tests in two ways. One is “sensitivity” which, in this case, is the ability of the test to identify those people who have developed antibodies to the COVID-19 virus after actually being infected and avoid “false negative” results. Put simply, if the test is done on 100 people who have been previously infected, the sensitivity measures how many of those people have a positive test. The test that we are currently using reports a sensitivity of approximately 97%.
The other is “specificity” which is the ability of the test to recognize those people who were not infected and do not have antibodies to the COVID-19 virus. If their test came up positive it would be a false-positive result. If the test is done on 100 people who have never been exposed to the virus, the specificity measures how many of them have a negative test. The test we are currently using has a reported specificity of 99%.
This is a very good performance for a medical test. However, because there is still a very low prevalence of COVID-19 in the general population (estimated to be around 2-4% at present) if we test 100 people with this test, we would expect about 3 of them to have an actual positive test but we will also find 1 of the 100 has a false positive test. So each positive test has about a 75% likelihood (3 true positive plus 1 false positive) of actually indicating prior infection. Therefore, obtaining a second confirmatory test is highly recommended.
As the prevalence of prior infection in the population increases over time, the performance of the test will improve because the ratio of true positives to false positives will increase.
The short answer is: We don’t know.
There is a lot of uncertainty with this new virus but broad use of antibody tests and clinical follow-up will provide the medical community with more information on whether or not people with positive antibody tests are protected against reinfection and how long that protection might last. Many experts believe that there is likely to be at least some degree of protection for several months.
While the FDA recommends that health care providers continue to use serological tests intended to detect antibodies to SARS-CoV-2 to help identify people who may have been exposed to the SARS-CoV-2 virus or have recovered from the COVID-19 infection, patients should also be aware of the limitations of these tests.