Antibody tests might be wrong up to half the time, CDC says

(WVLT/CNN) -- The Centers for Disease Control and Prevention say that antibody tests used to tell if someone has been infected with COVID-19 might be wrong up to half the time.

"Antibodies in some persons can be detected within the first week of illness onset," the CDC says. However, it added that such tests are not accurate enough to use to make important policy decisions.

"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 says. That applies to the workplace, CNN reported.

The CDC said health officials who use antibody tests are encouraged by the CDC to use the most accurate test they can find, and they might need to test people twice.

"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.

CNN reported that tests give more accurate results if the disease being tested for is common to the public. If an infection has only impacted a small percentage, even a small margin of error in a test can be magnified. In other words, if just five percent of the population being tested has the virus, a test with more than 90 percent accuracy can still miss half the cases.

Even if the test works, the CDC warns that having antibodies might not protect you from the virus. "It cannot be assumed that individuals with truly positive antibody test results are protected from future infection," the CDC says in the updated guidelines. "Serologic testing should not be used to determine immune status in individuals until the presence, durability, and duration of immunity is established."