What is an Antibody Test? The antibody test, also known as a serology, is the third COVID-19 diagnostic test currently available. Instead of looking for the presence of the virus, this test detects antibodies that have been created as a result of a previous COVID-19 infection Antibodies are proteins that the human body produces specific to each disease. It is one-way that the immune system fights off the specific infection, however it is currently unknown if a person will actually develop immunity, how long this immunity lasts, and if it will help prevent future infections . How Many Different Antibody Tests Are Available and At What Cost? There are currently 61 antibody tests approved for immediate use by the FDA via the Emergency Use Authorization . Individual test prices range between $36-$300, with most tests costing under $150 . Additional costs may be incurred from laboratory, hospital, and physician fees. However, these costs are not passed on to the patient and instead are covered through public health insurance such Medicaid and Medicare, as well as private health insurances. Federal law requires private insurers, Medicare, and Medicaid to cover the cost of a COVID-19 test without any cost to the patient through the Families First Coronavirus Response Act and the Coronavirus Aid, Relief, and Economic Security Act . These acts also provide grants to the uninsured as a way to get tested for free. However, patients may be responsible to pay out of pocket fees if they choose to go to out-of-network providers. Who Should Get Tested With an Antibody Test? If COVID-19 infection is suspected and symptoms are present, is the patient is advised to get either a molecular or antigen diagnostic test. The antibody test should only be reserved for patients who do not have any symptoms present and have suspicion of a past COVID-19 infection from more than 14 days ago. Like any COVID-19 test available, no test result should replace social distancing guidelines. How Long Until Someone Gets Results Back? Antibody tests are available in a variety of settings that require samples to be taken by finger stick or blood draw. The sample can either be sent to a laboratory for analysis or can be analyzed right away at point of care. Laboratory tests use the enzyme-linked immunosorbent assay (ELISA) or chemiluminescent immunoassay (CIA), which are methods of detecting certain antibodies present in the body to determine past COVID-19 infection . Most laboratory tests require special large equipment to complete these analyses, which delay the return of results 1-3 days . What Do Someone’s Test Results Mean? An antibody test looks for specific proteins made by the immune system in response to a threat, such as a virus. Antibodies can help fight infections by binding to a virus or bacteria to either neutralize it or enhance the immune response against the foreign microbe. A positive antibody test indicates that the patient previously had COVID-19 and their body produced made antibodies against the virus. Antibodies can take several days or weeks to develop after the patient has had an infection and may stay in their blood for several weeks or more after recovery. Because of this, antibody tests should not be used to diagnose COVID-19. At this time, virologists and immunologists do not know if the presence of antibodies means that a patient will be immune to COVID-19 in the future. How Fast are Antibody Tests? There are some antibody tests available at point of care, such as Access Bio’s CareStart COVID-19 IgM/IgG antibody test. This type of test does not require special equipment or a laboratory. The blood sample can be collected and analyzed on site. Antibody tests provided at point of care utilize a lateral flow device to detect antibodies in the blood. They can provide same day results and only require a blood sample from a finger stick versus the blood draw required for the laboratory test . How Accurate Are the Results? The accuracy of each test is determined by a variety of factors. Each approved test has a different level of accuracy in sensitivity (how well a test can accurately detect a person with the antibodies or true positives) and specificity (how well a test can rule out a person who does not have antibodies for COVID-19 or true negatives). Unfortunately, the accuracy of the antibody test depends on an individual’s body’s ability to develop antibodies after a COVD-19 infection and maintain those levels. Certain individuals who have been infected with COVID-19 may not develop detectable levels of antibodies. Those who do develop detectable levels can even have their levels diminish overtime to undetectable levels, further adding to the uncertainty of immunity from COVID-19 . Antibodies can stay in the blood for several weeks to months. However, it is currently unknown as to the exact time frame of antibody reduction and whether a person actually develops immunity to COVID-19 that can help prevent future reinfections . When Should Someone Get Tested Using an Antibody Test? Like other tests available for COVID-19, the timing of testing plays a significant role in the level of sensitivity and specificity. Antibody tests demonstrate the lowest level of sensitivity at Days 0-7 after the onset of symptoms and increase in subsequent days, with the highest level of accuracy observed after 14 days from the onset of symptoms . This is because It can take 1-3 weeks after a COVID infection for the body to produce antibodies . As a result, antibody tests are not a suitable method to detect current infections. Instead, an antibody test is only useful to determine if a person has had a past infection with COVID-19 since the infection may have been cleared by the time a person is tests positive on an antibody test.
Prepared in collaboration with graduate students from Keck Graduate Institute (Claremont, CA): Srbuhi Poghosyan, Diego Salinas, Larisa Malak Stepanian, Michael Thompson
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