Antibody tests are not used to diagnose a current COVID-19 infection. They look for evidence in a patient’s bloodstream that the person has been exposed to COVID-19 in the past.
Written by: Matt WindsorMedia contact: Adam Pope
Jose Lima, M.D. (right), and members of his immunology lab with an initial diagnostic report from the antibody test. Image courtesy Sherri Polhill.On Friday, April 24, 2020, the University of Alabama at Birmingham began deploying testing for antibodies against SARS-CoV-2, the novel coronavirus that causes COVID-19.
Jose Lima, M.D., director of the UAB Immunology Lab in the Department of Pathology, explains these tests and how they will be used in the fight against COVID-19.
COVID-19 diagnoses are made by searching a patient sample for genetic material unique to the SARS-CoV-2 virus.
Antibody tests look for evidence in a patient’s bloodstream that the person has been exposed to COVID-19 in the past. They are not used to diagnose a current COVID-19 infection.
The human immune system responds to an invading virus by producing new antibodies that specifically recognize that virus and help fight it off. Once the virus is gone, these antibodies remain in the bloodstream — standing guard so they can wipe out the virus quickly if it invades again.
The test being used at UAB, made by Abbott, “measures immunoglobulin G, or IgG, one of the classes of antibody” made by the immune system, said Lima, an assistant professor of laboratory medicine in the Department of Pathology. Lima’s team is responsible for antibody testing at UAB.
Antibody testing at UAB requires about one tube of blood, or 5 milliliters. The testing itself is done on the blood serum — the liquid portion of the blood — which is why these tests are also known as serology tests.
COVID-19 diagnosis tests, by contrast, are done on samples obtained by pushing a swab far back in the nose, a hotspot for the virus’s attack on the respiratory system.
Lima’s lab can test up to 200 antibody samples per hour, with a turnaround time of 24 hours for test results.
A positive result on antibody testing means that the patient’s blood contains IgG reactive against SARS-CoV-2. This is a very strong sign that the person has been exposed to the novel coronavirus.
In the validation process that led up to launching antibody testing, Lima and his team ran tests on blood samples from patients known to have COVID-19. They also ran tests on blood drawn well before COVID-19 appeared at the end of 2019. These tests showed numbers very similar to those seen in Abbott’s own validation tests, Lima noted.
In those Abbott tests, 100 percent of samples from 73 patients with positive SARS-CoV-2 diagnoses had a positive result for IgG 14 days after the onset of symptoms. (There is a small likelihood that a positive IgG result may be due to past or present infection with other common coronavirus strains.)
In tests done on 997 samples taken before September 2019, only four showed IgG reactivity to SARS-CoV-2, for a specificity of 99.6 percent.
“We feel confident this is a good test,” Lima said. “We shouldn’t see many false positives or negatives, although they can occur. The bigger concern is there is no correlation with immunity as of right now. We cannot assume that, once a patient has tested positive, that equates to immunity or protection.”
“We feel confident this is a good test,” Lima said. “We shouldn’t see many false positives or negatives, although they can occur. The bigger concern is there is no correlation with immunity as of right now. We cannot assume that, once a patient has tested positive, that equates to immunity or protection.”
“We are very limited in the conclusions we can draw from these tests,” Lima said. “That’s the message we need to send.”
COVID-19 is so new that “we still don’t know the correlation between having antibodies and immunity,” Lima said. Even if a patient turns out to have antibodies against the SARS-CoV-2 virus, “we can’t say they’re even partially protected or if they’re going to have transient immunity. At this point, we can only say that we have some degree of confidence that whoever tested positive was infected with the SARS virus. Other than that, I don’t think we can infer much at this point.
“We will have that data, but it’s not here yet.”
“We are allowing our physicians to order this test as they wish,” Lima said. Good candidates for antibody testing, he noted, include two groups:
Doctors should not use antibody testing as the sole basis for a COVID-19 diagnosis or to rule out a COVID-19 infection, the Department of Pathology noted in a letter to UAB clinicians. Antibody tests also should not be used to determine if a person has active COVID-19 infection, or to inform decisions regarding need for PPE.
Other uses of antibody testing are suggested:
At this point, antibody testing “is not the magic bullet,” Lima said. “The information we get from it is very limited and should be interpreted with caution.”
For health care providers, “it’s not going to translate to giving that peace of mind to someone who has tested positive” that they do not need to wear PPE or that they can care for sick patients without the concern of getting sick, Lima said. “A positive test doesn’t tell them that they can relax.”
For more information, visit uab.edu/coronavirus.
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