Covid-19 Antigen


The Food and Drug Administration (FDA) recently authorized the first antigen test to diagnose infection with SARS-CoV-2, the virus that causes COVID-19, so you’ve heard of it. The new test is from a company called Quidel Corp., but the FDA hopes to authorize additional tests of this type in the near future.

If you or your child have ever had a rapid strep test, you have had the first-hand experience with an antigen test. While other diagnostic tests for COVID-19, known as PCR tests, look for genetic material from the virus, the antigen test looks for molecules on the surface of the virus. PCR tests require expensive and specialized equipment and can take hours or days to obtain results. In contrast, antigen tests can be done in a lab or doctor’s office in about 15 minutes.

Unfortunately, there is a trade-off between speed and accuracy, and antigen testing often fails to identify people who are actually infected, a deficiency recognized by the FDA. “Positive antigen test results are very accurate,” writes the FDA, “but there is a higher probability of false negatives, so negative results do not rule out infection.” Just as your doctor may order a throat culture to permanently rule out strep throat when a rapid strep test is negative, the FDA recommends using the more sensitive COVID-19 molecular test for symptomatic individuals who test negative for strep. antigen “before making treatment decisions or to prevent the possible spread of the virus due to a false negative.”

Some public officials anticipate the use of the COVID-19 antigen test as a tool for widespread screening, including screening for asymptomatic people. Given the speed of the test and the relatively low cost, it is thought that people could be screened quickly and easily as a prerequisite for attending school, working, or traveling. But many medical experts, including Dr. Shawn Ferullo, MIT Medical’s chief of student health, believe that the test’s lack of sensitivity precludes such use.

However, he says, MIT Medical will evaluate this test, along with others, as they come on the market. “I think it’s safe to say that we would use antigen tests if they are available and show good data,” he says. “I imagine it will be a useful screening tool for patients with symptoms, in the same way, that we use rapid strep tests or rapid flu tests. We know that all of these rapid tests miss people who are really sick, but a positive result is almost always correct, and a more sensitive test is available as a follow-up for symptomatic patients who test negative. “

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