How important is it to have an antibody test? 3:31

. – Antibody tests used to determine if people have been previously infected with covid-19 could give wrong results up to 50% of the time, the US Centers for Disease Control and Prevention said. USA in a new guide published on their website.

Antibody tests, often called serological tests, look for evidence of an immune response to infection. “Antibodies in some people can be detected within the first week of disease onset,” says the CDC.

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They are not precise enough to make important political decisions, the CDC said.

“The results of the serological tests should not be used to make decisions about the grouping of people who reside or are admitted to congregational settings, such as schools, dormitories, or correctional facilities,” says the CDC.

“The results of serological tests should not be used to make decisions about the return of people to the workplace,” they added.

Health officials or healthcare providers who use antibody tests should use the most accurate test they can find and may need to screen people twice, the CDC said in the new guide.

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“In most parts of the country, including areas that have been heavily affected, the prevalence of the SARS-CoV-2 antibody is expected to be low, with a range of less than 5% to 25%, by so testing at this point could result in relatively more false positives and fewer false negative results, ”the CDC said.

The higher the sensitivity, the fewer false negatives a test will give. The higher the specificity, the less false positives. In all populations, the tests give more accurate results if the disease being analyzed is common in the population. If an infection has only affected a small percentage of people, even a very small margin of error will be increased in one test.

If only 5% of the tested population has the virus, a test with more than 90% accuracy can still miss half of the cases.

It’s a point that public health experts have made frequently in the past few weeks, but the CDC explains the problem in new advice on antibody testing.

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False positive, false confidence

A false positive will make someone believe that they have been infected when in fact they have not. There is little evidence now about whether being infected gives people immunity to a subsequent infection, but doctors fear that people will behave as if they were immune if they get a positive antibody test.

“It cannot be assumed that people with truly positive antibody test results are protected from future infections,” says the CDC in the updated guidelines. “Serological tests should not be used to determine immune status in individuals until the presence, durability, and duration of immunity are established.”

The new CDC guidelines echo the advice of groups like the University of Minnesota, which warns against using antibody tests to make political decisions.

The CDC explains why tests can give incorrect results so often. It has a lot to do with how common the virus is in the population being tested. “For example, in a population where the prevalence is 5%, a test with a sensitivity of 90% and a specificity of 95% will yield a positive predictive value of 49%. In other words, less than half of the positive tests will actually have antibodies, “the CDC explained.

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“Alternatively, the same test in a population with an antibody prevalence greater than 52% will yield a positive predictive prognosis greater than 95%, meaning that less than one in 20 people with a positive result will have a false positive result.”

Therefore, it is best to use tests with high specificity, which are unlikely to yield many false positives, and in populations where physicians suspect there are many cases, “the CDC said.

The Food and Drug Administration has also warned about the accuracy of antibody tests.