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Antibody testing.

https://www.fda.gov/medical-devices/...ing-sars-cov-2

General FAQs

Q: What is the difference between the types of tests available for SARS-CoV-2?
A: "Nucleic acid amplification tests," or "NAAT" tests are molecular tests that detect the virus's genetic material in a sample that typically comes from a patient's respiratory system. FDA-authorized NAAT tests for SARS-CoV-2 meet the EUA statutory standard, and based on the current available data, we believe are highly accurate. This means that a positive or a negative result from a NAAT test is likely to be true.

Another type of test, called a serology or antibody test, measures the amount of antibodies present in the blood when the body is responding to a specific infection, like COVID-19. This means the test detects the body's immune response to the infection caused by the virus rather than detecting the virus itself. In the early days of an infection when the body's immune response is still building, antibodies may not be detected. This limits the test's effectiveness for diagnosing COVID-19 and why it should not be used as the sole basis to diagnose COVID-19.

In response to an infection, such as COVID-19, the body develops an overall immune response to fight the infection. One component of the immune system's response is development of antibodies that attach to the virus and help eliminate it. The body's initial immune reaction produces general antibodies that attack many infections, called "IgM" antibodies. IgM antibodies indicate an active or recent infection. Because it takes time for the body to make IgM antibodies in response to SARS-CoV-2, their absence does not mean that someone is not infected. A test for IgM antibodies may give a false negative result in a patient with SARS-CoV-2, particularly early in infection. A patient may have a negative result early in infection even when they are symptomatic or asymptomatic but actively shedding the virus. Since IgM antibodies may not develop early or at all in infected patients, this type of antibody test is not used to rule out SARS-CoV-2 in an individual.

Over time, the body develops a second type of antibody in response to the infection that is more specific to the virus, called "IgG" antibodies. Most antibody tests detect IgG antibodies. On average, IgG antibodies take about 4 weeks to develop, but the time to development may vary substantially, and there is still a lot we do not know about SARS-COV-2. Since IgG antibodies generally do not develop until several weeks after infection, this type of antibody test, even though it is more specific to SARS-CoV-2, is not used to rule-out SARS-CoV-2 infection in an individual.

We also do not know how long IgM or IgG antibodies to SARS-CoV-2 will remain present in the body after the infection has been cleared.

More information on serology tests for SARS-CoV-2 can be found in the Serology QA section of this FAQ page.

Serology/Antibody Test FAQs


Q: If antibody tests are not used for diagnosis or exclusion of COVID-19 infection, what is their purpose? (Updated 4/17)
A: Serology tests that detect antibodies may detect different types of antibodies. The most common are IgM and IgG. A positive result from an appropriately validated serology test that detects IgM is likely to indicate that someone currently has or has recently had the virus. But a serology 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). Thus, antibody tests by themselves are of limited value in the immediate diagnosis of a patient where COVID-19 infection is suspected. 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.

Serology tests are of limited value in the immediate diagnosis or screening of a patient where COVID-19 infection is suspected because they cannot rule out presence of the virus. But positive results from appropriately validated serology tests that are designed to be very specific to the SARS-CoV-2 virus can confirm either that a patient has (for IgM antibodies), or more likely has recovered from (for IgG antibodies) a 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 been infected or exposed 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 been exposed to SARS-CoV-2 virus and have developed an 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. In addition, these test results can aid in determining who may 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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