New Cochrane review assesses how accurate antibody tests are
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The immune system of people who have COVID-19 responds by developing antibodies that attack the virus. Detecting antibodies in people’s blood may indicate whether they currently have COVID-19 or have had it previously. Whilst detecting current infection is usually done using swab tests within the first 5 days of illness, they may miss infection and are not available to all.

The studies looked at three types of antibody, IgA, IgG and IgM. Most tests measured both IgG and IgM, but some measured a single antibody or combinations of the three antibodies. There were not enough data to compare the accuracy of different tests.

-- The researchers found that the sensitivity of antibody testing is very closely related to when the test is performed.
-- Tests of the IgG and IgM antibodies at 8 to 14 days after onset of symptoms correctly identified only 70% of people who had COVID-19.
-- However, when the researchers looked at data reported at between 15 and 35 days after symptoms first began, antibody tests accurately detected over 90% of people with COVID-19.
-- There are insufficient studies to estimate the sensitivity of antibody tests beyond 35 days after the beginning of symptoms.
-- The tests only wrongly diagnosed COVID-19 in 1% to 2% of people without COVID-19.

In a population where COVID-19 was more common there would be more false negatives and fewer false positives.

Studies showed that antibody tests may have a role in diagnosing COVID-19 in patients who have had COVID-19 symptoms for two or more weeks but who have not had a swab (PCR) test or tested negative despite COVID-19-like symptoms.

Source: https://www.cochrane.org/news/new-cochrane-review-assesses-how-accurate-antibody-tests-are-detecting-covid-19
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