In places where COVID-19 prevalence is low, "less than half of those testing positive" for SARS-CoV-2 antibodies will actually have antibodies, according to new guidance from the Centers for Disease Control. The agency noted such tests should not play a factor in determining policies around reopening states.
"In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from less than 5% to 25%, so that testing at this point might result in relatively more false positive results and fewer false-negative results," the CDC said in the guidelines posted to its website over the weekend.
Antibody or serological tests look for the body's past immune response to an infection, unlike antigen testing, which can diagnose an acute infection whether or not antibodies are present. With COVID-19, antibodies are able to be detected in the blood about one to three weeks after symptom onset, according to the CDC.
Although it is likely that the presence of antibodies suggests decreased infectiousness and provide some degree of immunity from future infection, the CDC advises that more data is needed to modify public health recommendations based on serology test results alone.
"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. "Serologic test results should not be used to make decisions about returning persons to the workplace."
People also shouldn't assume that those with COVID-19 antibodies are safe from future infection, "until the presence, durability, and duration of immunity is established," the CDC said.
The CDC also suggests several strategies to improve the probability that subjects with a positive test truly have the antibodies, including focusing testing on persons with a history of COVID-19-like illness and choosing tests with at least 99.5% specificity, or the rate it correctly identifies those without the disease.