Antibody Tests: What are They? Do You Need One?
Many commercial antibody tests have been made widely available to the public in recent months for healthcare professionals to track the spread of the virus and for individuals to see if they have been exposed to SARS-CoV-2 in the past. With so many tests available, you might be wondering if you should get an antibody test, why they are important, and what they can and can’t be used to determine.
What are antibodies?
To understand how antibody tests are used by medical professionals to both identify and track the virus in individuals and populations, it is vital to understand the immune system and how antibodies work. Antibodies are proteins produced by B cells in your body. When exposed to a pathogen, your immune system breaks down the foreign invader and presents the antigens, proteins specific to the pathogen, to the B-cells in your body. The B-Cells then differentiate into plasma cells and memory cells. Plasma cells produce antibodies that allow your immune system to identify and attack the invader, and usually takes 1-2 weeks to elicit a full immune response. During these two weeks, you will feel the sickest. After antibody levels have peaked and symptoms cease, a doctor may determine the patient has clinically recovered, even if the patient still tests positive on an RNA amplification or antigen tests. On the other hand, memory cells are long-living cells that can remember pathogens that have already infected the body in the past and can elicit a faster immune response and create the antibodies to that pathogen extremely quickly granting “immunity” to the pathogen in the future.
Does having antibodies make me immune to coronavirus?
As with most viruses, coronavirus infection produces a standard immune response and antibody reaction, so you can easily test to see if you have been infected in the past with an antibody test. However, not all immune responses, despite relying on antibodies, result in future immunity. While antibodies produced after an infection grant immunity to past types of coronavirus such as SARS, more research must still be conducted to determine the extent of how contracting COVID-19 results in conferred immunity.
However, just because antibody levels vary in a previously infected individual, or that antibody levels have declined after recovering does not mean that contracting the virus does not confer future immunity. While antibody levels may decrease after infection, this is typical for most viral diseases. If you are exposed again, you will not have antibodies immediately available to fight the pathogen. However, memory cells that have been produced will elicit a rapid immune response, neutralizing the infection before you get sick. Most research has shown that most of those previously infected with SARS-Cov-2 possess the neutralizing antibodies in their blood that enable them to fight off future infections.
Additionally, those who experienced a more severe response to the virus have been shown to have a more robust antibody count after infection. Current studies suggest that those exposed to the novel coronavirus will have acquired immunity to the disease once recovered in most cases. Dr. Florian Krammer of the Icahn School of Medicine at Mt. Sinai in New York has said, "It's reasonable to assume that there will be protection for a time frame of one to three years”. What is important to understand is that while more research must still be conducted in order to validate the extent of and how long recovered individuals are immune, those who have been infected possess the ability to fight the virus effectively in the future. What is even more important is how this information should be interpreted; antibody in most cases. Testing is a vital part of not only confirming whether or not an individual has been previously infected.
How are antibody tests used?
An antibody test can be administered to check for the presence of antibodies in your bloodstream that indicates a past infection of the virus that causes COVID-19. This test is usually administered by taking a small blood sample.
The assay - the testing mechanism - can confirm the presence of antibodies for the novel coronavirus (SARS-CoV-2) within your body within 10-15 minutes. Most antibody assays developed by leading labs such as Abbott, Roche, and Beckman Coulter can test for both IgM and IgG antibodies. IgM is typically developed rapidly during the onset of the infection, and this is the first type of antibody to start appearing in the blood typically 3-10 days after the initial infection. On the other hand, IgG IgG is usually produced 6-14 days after being infected and lasts longer in the blood. According to the CDC, however, it is rare to test positive for IgM and not IgG as well, as most patients develop both antibodies simultaneously in most cases of SARS-CoV-2 infection. After antibody levels have peaked and symptoms cease, a doctor may determine the patient has clinically recovered, even if the patient still tests positive on an RNA amplification or antigen tests. Unlike a titer test, most of these tests are qualitative; these tests only determine whether or not viral antibodies are present in your blood or not, not the actual quantity. This is an important distinction because the antibody level can be a useful metric to use when examining how severe a reaction an individual has experienced, and how they will rebound from the disease, essential factors when determining if a patient is clinically recovered or not. These tests are useful for both a patient’s understanding of if they have been exposed to SARS-CoV-2, and for professionals tracking the epidemiology - the study of how a virus or disease spreads through a population - of the virus in order to understand it better.
Do I need an antibody test? Are antibody tests effective?
Antibody tests should be used as a way for scientists and healthcare professionals to track the spread of a virus and so individuals can know if they have been exposed in recent months. They should not be used to replaced PCR or antigen testing to determine acute infection of the virus. In most cases, antibody tests are only effective when an individual has produced the antibodies specific to the virus, typical 1-3 weeks following infection.
Most assays available today have a very Positive Predictive Value (PPV), which is the probability that someone with positive test results actually possesses the antibodies. The prevalence of the disease- the number of cases of a disease in a given population - and the assay type, can influence this value. For instance, tests developed by Beckman Coulter have a very high PPV even in low disease prevalence rates. For example, at a 3.5% prevalence rate, PPV is 90.1%; at a higher PPV, like 16%, the PPV is 97.9%. Most assays developed by major diagnostic laboratories meet similar standards and meet FDA requirements for accuracy and specificity. Almost all antibody tests on the market have a 95% confidence interval of greater than 97% PPV, meaning that 95% of the time, the tests will be roughly 97% accurate across all diagnoses.
It is essential to understand that antibody tests do not and will not replace direct detection methods, which are used to determine if an individual is acutely infected with SARS-CoV-2.
Unlike direct detection methods such as viral nucleic acid amplification or antigen detection tests, antibody tests are used to discern whether the individual being tested was previously infected—even if that person never showed symptoms. Suppose you have been recently exposed to SARS-CoV-2, or have been in close contact with someone who tested positive for COVID-19. In that case, an antibody test will not be sufficient to diagnose you, and you need to be tested at a test center/medical clinic or doctor.
More research is needed to confirm how an antibody response contributes to a lasting immunity, if at all. Because antibody levels can vary after infection, and it is unknown how this virus will mutate over the next year, it cannot be determined with certainty at the moment if antibody production is is key to being immune to the virus. If you test positive on an antibody test, you should continue being cautious about the virus by social distancing, wearing a mask, and making sure you are sanitary. While antibody testing is a useful tool to track epidemiological trends and to see if individuals might have been exposed in the past to SARS-CoV-2, individuals who have been in close contact someone with COVID-19 should be tested at a clinic using a PCR or antigen test to immediately determine whether or not an individual is presently infected with the novel coronavirus. Ultimately antibody tests are useful tools for understanding the spread of infection across the country, as well as for seeing how immune responses determine overall morbidity and mortality of infected and recovered individuals. While antibodies present in a patient’s blood suggest immediate resistance to re-infection, more research must be conducted to learn more about how immunity to the virus can be achieved.
Sources:
https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html
https://www.cdc.gov/coronavirus/2019-ncov/testing/serology-overview.html
https://www.gavi.org/vaccineswork/how-do-we-know-who-immune-covid-19
https://abcnews.go.com/Health/covid-19-experts/story?id=71792664
https://kidshealth.org/en/parents/test-immunoglobulins.html
https://achi.net/newsroom/defining-covid-19-terms-viral-shedding/
