Info from the FDA: https://www.fda.gov/medical-

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Serology/Antibody Test FAQs
Q: Are antibody, or serology, tests used to diagnose SARS-CoV-2 infection? (Updated 5/4)
A: The terms "serological" or "antibody" tests are generally used to refer to tests that detect antibodies to the SARS-CoV-2 virus. Because the antibodies are part of the body's immune response to exposure and not the virus itself, such testing cannot be used for diagnosis of infection. Based on the underlying scientific principles of antibody tests, we do not expect that an antibody test can be shown to definitively diagnose or exclude COVID-19 infection. SARS-CoV-2 antibody tests are intended for use as an aid in identifying individuals with an adaptive immune response to SARS-CoV-2, indicating recent or prior infection, by detecting antibodies to SARS-CoV-2 in human blood specimens.

As stated in the Policy for Coronavirus Disease-2019 Tests, validated antibody tests offered under the policy in that guidance should, among other things, include in instructions for use and test reports information to convey that negative results do not preclude acute SARS-CoV-2 infection and that, if acute infection is suspected, direct testing for SARS-CoV-2 is necessary. SARS-CoV-2 serology tests should be ordered only by clinicians who are familiar with the use and limitations of the test.

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Q: If antibody tests are not used for diagnosis or exclusion of COVID-19 infection, what is their purpose? (Updated 5/4)
A: Serology (antibody) tests may detect different types of antibodies. The most common are IgM and IgG. High quality serological tests can help us understand whether a person or population of people have developed antibodies indicative of an adaptive immune response to COVID-19.

Because a serology test can yield a negative test result even in infected patients (e.g., if antibody has not yet developed in response to the virus) or may be falsely positive (e.g., if antibody to a coronavirus type other than the current pandemic novel strain is present), antibody tests should not be used in the immediate diagnosis of a patient where COVID-19 infection is suspected. That is, these tests should not be used to diagnose acute COIVD-19 infection. Using this type of test on many patients may help the medical community better understand how the immune response against the SARS-CoV-2 virus develops in patients over time and how many people may have been infected. While there is a lot of uncertainty with this new virus, it is also possible that, over time, broad use of antibody tests and clinical follow-up will provide the medical community with more information on whether or not, and how long, a person who has recovered from the virus is at lower risk of infection if they are exposed to the virus again.

Positive results from appropriately validated serology tests that are designed to be very specific to the SARS-CoV-2 virus can indicate whether a patient has had recent or prior COVID-19 infection. In addition, although not everyone who is infected will develop an antibody response, appropriately validated serology tests, when used broadly, can be useful in understanding how many people have developed an adaptive immune response to the virus and how far the pandemic has progressed.

Serology tests can play a critical role in the fight against COVID-19 by helping healthcare professionals identify individuals who have antibodies to SARS-CoV-2 virus and have developed an adaptive immune response. In the future, this may potentially be used to help determine, together with other clinical data, whether these individuals may be less susceptible to infection. At this time, it is unknown for how long antibodies persist following infection and if the presence of antibodies confers protective immunity. In addition, these test results can aid in determining who may be eligible to donate a part of their blood called convalescent plasma, which may serve as a possible treatment for those who are seriously ill from COVID-19.

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