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, UAB began deploying testing for antibodies against SARS-CoV-2, the novel coronavirus that causes COVID-19.
We asked Jose Lima, M.D., director of the UAB Immunology Lab in the Department of Pathology, to explain these tests and how they will be used in the fight against COVID-19.
1. Antibody tests look at your body’s response to SARS-CoV-2. They are very different than the “coronavirus tests” used to diagnose patients with COVID-19, which look for signs that the virus is present in your body.
COVID-19 diagnoses are made by searching a patient sample for genetic material unique to the SARS-CoV-2 virus.
Antibody tests aren’t 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.
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 that 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, explained Lima, an assistant professor of laboratory medicine in the Department of Pathology. Lima’s team is responsible for antibody testing at UAB.
UAB is using the Architect antibody test from Abbott. Image courtesy Abbott.
2. These are blood tests — not nose swabs.
Antibody testing at UAB requires about one tube of blood — 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 micrograph of the novel coronavirus SARS-CoV-2, which causes COVID-19. Note the yellow blobs on the outer surface of the virus — the spike proteins that allow it to attach to and invade human cells. Image courtesy NIH.
A 3-D printed model of the coronavirus (in back), with spike proteins in red, and a model of one of the spike proteins (foreground). Image courtesy NIH.
UAB's antibody test looks for the presence of immunoglobulin G (IgG), illustrated above, in a patient's bloodstream. IgG attaches to the spike protein of SARS-CoV-2 via the two foreign particle binding sites on its upper "arms." Image courtesy National Library of Medicine.
3. Antibody tests let you know if you have been exposed to SARS-CoV-2. They don’t tell you if you are “immune.”
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 very similar numbers to those seen in Abbott’s own validation tests, Lima noted.
| “We still don’t know the correlation between having antibodies and 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…. We will have that data, but it’s not here yet.” |
In those Abbott tests, 100% 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%.
“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 that equates to immunity or protection.”
4. Here’s what a positive test DOES NOT mean.
- Having a positive antibody test does not necessarily indicate that an individual is protected against re-infection. Antibody tests give a reading on the amounts of antibodies in a patient’s blood.
- Reactive IgG results do not indicate or rule out active infection or asymptomatic carriage. It can take 2-3 weeks, as noted earlier, for the IgG antibodies to develop. It is not yet known whether people who are carriers of SARS-CoV-2 develop an immune response against the virus or not.
- Nonreactive IgG results do not rule out active SARS-CoV-2 infection or indicate that a person may be an asymptomatic carrier, for the reasons noted in the two bullets above.
“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,” Lima said. “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.”
5. So who should be tested?
“We are allowing our physicians to order this test as they wish,” Lima said. Good candidates for antibody testing, he noted, include two groups:
- patients who have signs and symptoms of COVID-19 and are suspected to have the disease but may have had negative viral RNA testing; and
- patients who may have been infected in the past and are trying to confirm that they had COVID-19.
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. And they should not be used to inform decisions regarding need for PPE.
Other uses of antibody testing are suggested:
- Identify patients who have recovered from COVID-19 who may be potential donors of convalescent plasma. Plasma containing antibodies against SARS-CoV-2 is being used at UAB and elsewhere as a treatment for people seriously ill with COVID-19.
- Conduct epidemiologic studies to gather data on the prevalence of COVID-19 in a community or regions.
- Verify vaccine response once a correlation between antibodies and protection has been identified.
6. Antibody testing is not a magic bullet.
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 don’t 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.”
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