COVID-19 Antibody Tests May Be Wrong Half the Time: CDC

May 27, 2020 Updated: May 27, 2020

The Centers for Disease Control and Prevention (CDC) said in new guidance that COVID-19 antibody tests could be wrong half the time and so should not be relied on to guide policy decisions regarding the deadly disease.

Antibody tests, sometimes referred to as serological tests, look for evidence of an immune response to infection and so can be used to determine if someone was sick with COVID-19, the respiratory illness caused by the Chinese Communist Party (CCP) virus.

The CDC cautions, however, that antibody tests are not accurate enough to reliably guide policy.

“Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” the CDC said, adding that they should similarly not be used to make decisions about returning people to the workplace.

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A medical specialist takes an antibody test in Hadassah clinic at the Skolkovo innovation center on the outskirts of Moscow, Russia, on April 24, 2020. (Maxim Shemetov/Reuters)

The agency called for more studies into COVID-19 antibody tests, which have received Emergency Use Authorization (EUA) by the Food and Drug Administration (FDA).

“Additional data are needed before modifying public health recommendations based on serologic test results, including decisions on discontinuing physical distancing and using personal protective equipment,” the CDC said.

Separately, the FDA cautioned that antibodies may not be detected in the early stages of COVID-19 infection, when the body’s adaptive immune response is still building.

“This limits the test’s effectiveness for diagnosing COVID-19, and this is one reason serology tests should not be used as the sole basis to diagnose COVID-19,” the FDA said.

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A health worker wears a protective mask and suit as she extracts blood from a patient to perform an antibody test for COVID-19 at the Dworska Hospital in Krakow, Poland, on April 9, 2020. (Omar Marques/Getty Images)

To minimize the risk of a false positive when using a COVID-19 antibody test, the CDC recommends choosing a test with high specificity and selectively testing populations and individuals with an elevated likelihood of previous exposure to the deadly bug.

Another measure that will increase accuracy when using serological tests is by using two tests, one after another, when the first test yields a positive result.

The CDC, in its guidance, outlined the limitations of antibody tests.

“Serologic test results do not indicate with certainty the presence or absence of current or previous infection with SARS-CoV-2,” the CDC said, using the scientific term for the virus.

Also, “the presence of antibodies cannot be equated with an individual’s immunity from SARS-CoV-2 infection,” the agency added.

Inaccuracy in serological testing is impacted by how common the virus is within the population, the CDC said. If the deadly bug infected only a small number of people who were later given an antibody test, it will magnify the margin of error.

The implication is that even a test with more than 90 percent accuracy can still be wrong more than half the time if a small percentage of the population has been infected.

“In a population where the prevalence is 5 percent, a test with 90 percent sensitivity and 95 percent specificity will yield a positive predictive value of 49 percent. In other words, less than half of those testing positive will truly have antibodies,” the CDC said.

The agency also outlined other limitations of serological testing, including that results might be skewed by the presence of other kinds of coronavirus or that antibodies could disappear over time.

“Some tests may exhibit cross-reactivity with other coronaviruses, such as those that cause the common cold. This could result in false-positive test results. Some persons may not develop detectable antibodies after coronavirus infection. In others, it is possible that antibody levels could wane over time to undetectable levels,” the CDC said.

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