Coronavirus Antibody Tests Can Give Inaccurate Results Half The Time, Warns CDC

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While it has been suggested that recovering from the virus could provide a person with short-term immunity, the agency urged people who test positive for the antibodies to not assume that they are protected from getting the coronavirus again.

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. The timing of antibody tests can affect the result as well; as the CDC notes, the most useful antibodies for assessing antibody response are not present early in infection, and only become detectable 1-3 weeks after symptom onset.

In a hypothetical population where 5% of people had antibodies, antibody tests might have a "positive predictive value" of just 49%, the CDC said. But the CDC says that these tests aren't accurate enough to make important policy decisions. It added that the results also shouldn't be used "to make decisions about returning persons to the workplace".

"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", said the CDC.

Aside from significantly infected places like food processing plants experiencing COVID-19 outbreaks, the CDC said, antibody tests are more likely to show false positives than false negatives. Since the EEOC has largely deferred to the CDC on this issue, employers who condition an employee's return to work on a positive test for antibodies may be subject to claims by both the individual and the EEOC.

The higher the sensitivity, the fewer false negatives a test will give. A lot has to do with how common the virus is in the population being tested. If an infection has only affected a small percentage of people being tested, even a very small margin of error in a test will be magnified. "In other words, less than half of those testing positive will truly have antibodies", the CDC said.

"Some tests may exhibit cross-reactivity with other coronaviruses, such as those that cause the common cold". In such instances, there could be false-positive test results.

It is also possible that the antibody levels in people could sometimes disappear over time and go undetected.



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