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Antibody tests are intended to determine if you've previously been infected with the coronavirus, regardless of whether you experienced COVID-19 symptoms. With Kroger introducing rapid antibody testing at all its pharmacies (cost: $25), it's crucial to understand the scope and limitations of these tests.
A positive result for COVID-19 antibodies might lead you to believe you're immune and can ease up on safety measures. However, this is unwise—the CDC advises maintaining the same precautions irrespective of antibody test outcomes. The test may be inaccurate, and even if correct, there's insufficient evidence to confirm immunity against future infections.
Interpreting test results requires context
Interpreting test results requires careful consideration of context. “In medicine and laboratory testing, there’s a concept known as pretest probability,” explains Dr. S. Wesley Long from Houston Methodist Hospital. “It’s about assessing the likelihood that a patient has the condition being tested for.”
In simpler terms, if someone exhibited COVID-19 symptoms and tested positive for antibodies, the result is likely accurate. However, if a person isolated on a remote island, had no exposure to the outside world, and never showed COVID-19 symptoms but tested positive, the result might be less reliable. Given the novelty of these tests, context is crucial in interpreting results.
“If you’ve never experienced symptoms, be cautious in interpreting the results,” Long advises. Asymptomatic transmission remains poorly understood, so a positive result without prior symptoms calls for extra caution—don’t assume immunity without confirmation. And on the topic of immunity...
Does a positive test result guarantee immunity?
If you previously had COVID-19 symptoms and received a positive antibody test result, there’s a reasonable chance you may be immune. However, much about COVID-19 remains unknown, including the duration of any potential immunity.
“We currently lack sufficient data to confirm whether these antibodies provide true protection against reinfection or how long such protection might last,” Long explains. “While we believe they likely offer protection for many individuals, this remains uncertain.”
Additionally, a positive test result could be incorrect. Even highly accurate tests have a small margin of error, and false positives are particularly common with these types of tests.
For instance, the FDA estimates that a false positive on the Assure test (which appears to be the test Kroger uses) has an 80% chance of indicating actual antibodies, assuming 5% of the population in your area has had the virus. We break down the math here, but the key takeaway is that false positives are more frequent when true positives are rare. In most U.S. regions, less than 10% of the population has been infected.
Does a negative test mean I lack immunity?
It’s also possible to have a confirmed COVID-19 diagnosis and still test negative for antibodies. Some individuals, for reasons not fully understood, do not produce enough antibodies to yield a positive result.
A negative result despite a confirmed COVID-19 case could stem from a few reasons. One possibility is that the test’s sensitivity level failed to detect antibodies, even if they are present. This doesn’t necessarily mean you lack immunity—it could simply indicate that your antibody levels are below the test’s detection threshold.
Another possibility is that, for some reason, you didn’t develop immunity after being infected. While rare, there are cases where individuals, like children who contracted chickenpox multiple times, don’t gain lasting immunity despite prior infection.
Ensure your test is FDA-authorized
Before taking an antibody test, research the specific test thoroughly. “Not all antibody tests are equal,” Long notes, stressing the importance of choosing one approved by the FDA under Emergency Use Authorization. Start by checking the FDA’s website, which lists all EUAs issued for COVID-19, including serology tests.
Early in the pandemic, this was critical due to reports of substandard tests flooding the market. While this is less of an issue now, the FDA warned as recently as October that fraudulent tests, vaccines, and treatments remain a concern.
