Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers
Antibodies are developed by the body in response to an infection or after vaccination. SARS-CoV-2 is the virus that causes COVID-19. SARS-CoV-2 antibody tests detect antibodies to the SARS-CoV-2 virus. SARS-CoV-2 antibody tests can help identify people who may have been infected with the SARS-CoV-2 virus or have recovered from COVID-19. Antibody tests should not be used to tell you if you have an active COVID-19 infection.
Scientists continue to learn more about COVID-19 and COVID-19 immunity. At this time, SARS-CoV-2 antibody tests do not tell you if:
- You currently have COVID-19, the disease caused by the SARS-CoV-2 virus,
- You have immunity that will prevent COVID-19,
- You need a COVID-19 vaccine, or
- Your COVID-19 vaccine worked.
On this page:
- Antibodies and Antibody Tests: The Basics
- Antibody Tests: Not for Use to Check Immunity
- Antibody Tests: Results and Terms
- Additional Resources
Antibodies and Antibody Tests: The Basics
Q: What are antibodies?
A: Antibodies are proteins made by your body's immune system to help fight off infections, including those caused by viruses. Some antibodies in your body may protect you from getting those infections. Your immune system can also safely learn to make antibodies through vaccination. If antibodies give you this protection and how long this protection lasts can be different for each disease and each person. Antibodies are just one part of your immune response.
Q: Are antibody tests used to diagnose COVID-19?
A: No. An antibody test cannot be used to diagnose current COVID-19 because an antibody test does not detect SARS-CoV-2. Only COVID-19 diagnostic tests can be used to diagnose current COVID-19. A positive antibody test result can be used to help identify people who may have had a prior SARS-CoV-2 infection or prior COVID-19. An antibody test does not show if you have a current SARS-CoV-2 infection or COVID-19 because the antibodies are part of the body's immune response to infection, and antibody tests do not test for the virus itself. It also can take days to weeks after the infection for your body to make detectable antibodies.
Antibody Tests: Not for Use to Check Immunity
Q: Will a positive result on a SARS-CoV-2 antibody test mean I have immunity and I will not get COVID-19?
A: No. At this time, SARS-CoV-2 antibody tests do not tell you if you have immunity that will prevent you from getting COVID-19. A positive SARS-CoV-2 antibody test does not necessarily mean you are immune or have immunity that will prevent COVID-19. More research is needed to understand what SARS-CoV-2 antibody test results can tell us. And, SARS-CoV-2 antibodies detected in your blood reflect only one part of your immune system, which also includes T-cells and other components that are part of your body's immune response.
Q: Should I get a SARS-CoV-2 antibody test to decide if I need a COVID-19 vaccine or after I am vaccinated to see if my vaccine worked?
A: No. At this time, antibody test results should not be used to decide if you need a COVID-19 vaccine or a vaccine booster, or to determine whether your vaccine worked. There is not a clear connection between SARS-CoV-2 antibody test results, the need for a COVID-19 vaccine or booster, or whether a vaccine worked in a person. Also, some SARS-CoV-2 antibody tests may not detect the kind of antibodies created following vaccination. SARS-CoV-2 antibodies detected in your blood reflect only one part of your immune system, which also includes T-cells and other components that are part of your body's immune response.
More research is needed to understand the role of SARS-CoV-2 antibody testing in evaluating a person's immunity or protection against COVID-19 and understanding if antibody tests will be helpful for deciding if a person should receive a COVID-19 vaccine. If you have questions about whether a SARS-CoV-2 antibody test is right for you, talk with your health care provider or your state or local health department.
Q. Can a SARS-CoV-2 antibody test tell me if I could infect other people?
A: No. Antibody tests do not tell you whether or not you can infect other people with SARS-CoV-2. Current information indicates people infected with SARS-CoV-2 can still transmit the SARS-CoV-2 virus and infect other people, even if they are COVID-19 vaccinated or have detectable SARS-CoV-2 antibodies from a previous infection.
Antibody Tests: Results and Terms
Q: What does a positive SARS-CoV-2 antibody test result mean?
A: A positive antibody test result could mean you previously had a SARS-CoV-2 infection or COVID-19. A positive antibody test could also mean the test is detecting antibodies in your blood in response to your COVID-19 vaccine. Not all SARS-CoV-2 antibody tests will detect antibodies in response to a COVID-19 vaccine.
Q: What does a negative SARS-CoV-2 antibody test result mean?
A: A negative result on a SARS-CoV-2 antibody test means antibodies to the virus were not detected in your blood.
It is unknown if all people who have a SARS-CoV-2 infection will develop antibodies in their bodies in an amount that can be detected by a SARS-CoV-2 antibody test. Also, even if people do develop antibodies, the antibody levels may decrease over time to levels that can't be detected by a SARS-CoV-2 antibody test. It is also important to note that different antibody tests may detect different antibodies and different levels of antibodies.
A negative result could mean:
- You have not been infected with SARS-CoV-2 previously.
- You had a previous SARS-CoV-2 infection but:
- Your body did not make antibodies to the infection yet. It can take days to weeks after an infection for your body to make antibodies.
- Your body made SARS-CoV-2 antibodies but the level of antibodies in your sample is too low to be measured by the test that was used.
- You were vaccinated with a COVID-19 vaccine, but the antibody test does not detect the same kind of antibodies your body produced in response to your COVID-19 vaccine.
- The test result may be wrong, known as a "false negative." This occurs when the test does not detect antibodies even though you may have antibodies for SARS-CoV-2.
Q: What if I get different results on two SARS-CoV-2 antibody tests and they don't agree? Which one is right?
A: Results may be different for several reasons, including:
- The design of the tests - different antibody tests may detect different antibodies and different levels of antibodies.
- The performance of the tests, including the sensitivity and specificity of each test (see What do sensitivity and specificity mean in SARS-CoV-2 antibody testing below).
- The timing of when you took the tests, how long it may take for your body to develop antibodies after a potential SARS-CoV-2 infection, and whether antibody levels may decrease over time.
For this and other reasons, you should always review your test results with your health care provider.
Q: What is the difference between qualitative, quantitative, and semi-quantitative SARS-CoV-2 antibody tests? What do the reported numbers mean?
A: Qualitative, semi-quantitative, and quantitative tests all tell you if SARS-CoV-2 antibodies were detected in your blood sample with the specific test used. These tests report whether SARS-CoV-2 antibodies were detected or not detected over a certain threshold, and this threshold may vary between different SARS-CoV-2 antibody tests. Different antibody tests may also be designed to detect different SARS-CoV-2 antibodies in addition to the different levels of antibodies. This means that different tests may provide different results for the same blood sample.
- Qualitative SARS-CoV-2 antibody tests only report whether SARS-CoV-2 antibodies were detected or not detected and do not provide any indication of the amount of antibodies detected, other than whether it was above or below the detection threshold for that test.
- Semi-quantitative SARS-CoV-2 antibody tests also report the relative concentration of antibodies in a person's blood sample at the time of testing, meaning the level of antibody in your blood sample is assigned a number on a scale that applies only to that specific authorized test. The scale for each test is determined and validated by the test developer but is not comparable to results from any other SARS-CoV-2 antibody test, whether semi-quantitative or otherwise.
- Quantitative SARS-CoV-2 antibody tests produce results that are standardized because quantitative antibody tests are traceable to a certified reference material. This means that numeric results can be compared between different quantitative tests that are traceable to the same certified reference material. However, at this time, there is not enough scientific information to understand what the numerical results mean about an individual's protection from infection.
Q: What do sensitivity and specificity mean in SARS-CoV-2 antibody testing?
A: Sensitivity is the ability of the test to identify people with antibodies to SARS-CoV-2. A highly sensitive test will identify most people who truly have antibodies, and a small number of people with antibodies may be missed by the test (false negatives).
Specificity is the ability of the test to correctly identify people without antibodies to SARS-CoV-2. A highly specific test will identify most people who truly do not have antibodies, and a small number of people without antibodies may be identified as having antibodies by the test (false positives).
The FDA included information about test performance expectations for SARS-CoV-2 serology tests in the Emergency Use Authorization (EUA) serology templates. For information on authorized serology test performance, see EUA Authorized Serology Test Performance.
Q: What does predictive value mean in SARS-CoV-2 antibody testing?
A: Predictive values are probabilities calculated using a test's sensitivity and specificity, and an assumption about the percentage of individuals in the population who have antibodies at a given time (which is called "prevalence" in these calculations).
Positive predictive value is the probability that a person who has a positive test result truly has antibodies. Positive predictive values for SARS-CoV-2 antibody tests are impacted by how common SARS-CoV-2 antibodies are in the population being tested at a certain time.
The lower the prevalence, the lower the positive predictive value. This means that SARS-CoV-2 antibody tests used in areas with low prevalence (small number of people that have SARS-CoV-2 antibodies) will have a positive predictive value lower than in an area with higher prevalence.
Low positive predictive value may lead to more individuals with a false positive result. This could mean that individuals may not have developed antibodies to the virus even though the test indicated that they had. If a high positive predictive value cannot be achieved with a single test result, two tests may be used together to help identify individuals who may truly be SARS-CoV-2 antibody positive.
Negative predictive value is the probability that a person who has a negative test result truly does not have antibodies. Negative predictive values for SARS-CoV-2 antibody tests are also impacted by how common SARS-CoV-2 antibodies are in the population being tested at a certain time. Negative predictive value is higher in areas with low prevalence and lower in areas with high prevalence. This means that in areas where a lot of people have SARS-CoV-2 antibodies, a negative result is more likely to be a false negative result compared to the likelihood of a false negative result in areas where few people have SARS-CoV-2 antibodies. This could mean that individuals may have developed antibodies to the virus even though the test indicated that they had not.