How accurate are COVID-19 rapid antigen tests ?

Rapid antigen tests are quick tests that can be done in a pharmacy or at home as a self-test. They are recommended by experts because they can be done regularly at home at a low cost. They, therefore, play a role in controlling the epidemic.

Research shows that rapid tests are less reliable than the test performed by a professional and a positive result obtained from a self rapid test must be confirmed by PCR. Especially with the circulation of the Omicron variant.

Rapid test omicron at home

This is not the first reservation about this method. It was already known that the rapid test was negative at the beginning of the infection when the person was not yet showing any symptoms. This is when the person is actually most contagious.

It was also known that cold weather distorts the results of some antigenic tests. The microbiologist of the University of Antwerp, Herman Goossens, had demonstrated that the method could deliver false positives, linked to the temperature. This test should therefore be kept out of the refrigerator and performed indoors.

Despite these new reservations, U.S. authorities continue to allow the use of these tests, following the specific indications for use. “If a person tests negative with an antigenic test, but is suspected of having Covid-19, for example by having symptoms or a high probability of infection due to exposure, follow-up with a molecular test (PCR, Editor’s note) is important,” the FDA insists.

Rapid tests are less sensitive to Omicron

The Food and Drug Administration (FDA) has issued an advisory regarding the impact of Omicron on the reliability of covid rapid tests. The U.S. agency, along with the National Institutes of Health (NIH), studied the performance of rapid tests to detect Omicron. The good news is that they are able to detect Omicron, but with lower sensitivity than for other variants. In other words, rapid tests are more likely to provide a false negative (the test appears negative when coronavirus is present) with Omicron than with Delta.

Despite a negative self-test, the onset of typical Covid-19 symptoms should raise alarm and should be tested again, by PCR, to confirm the presence of coronavirus or not.

Fifteen thousand rapid antigen tests are currently performed every day.
Alexandra Koch, Pixabay

Rapid test effectiveness for Omicron

Covid rapid tests are antigenic tests, they detect the presence of a protein of SARS-CoV-2, the nucleocapsid, in the sample. PCR tests, on the other hand, look for the genetic material of the coronavirus, its RNA. With screening techniques, the PCR test can suspect the presence of the Omicron variant, which is impossible with a rapid test alone.

The decrease in sensitivity of rapid tests does not call into question their use according to the FDA. They are still useful for detecting coronavirus, especially in people with high viral loads in the nose. Some pharmacists propose to supervise the rapid tests to limit the errors of sampling or reading of the result.

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