The rapid COVID-19 screening tests that Quebecers are snapping up on work by detecting the antigens of the virus behind the disease, macromolecules that can trigger an immune reaction in the body.
“On the cellulose membrane, there are like antibodies, if you will, which are made to catch the combination of antigens, therefore the piece of virus that would be present in the nose of the infected person”, explains Dr. Judith Fafard , medical director of the Quebec Public Health Laboratory.
By following the instructions in the kit, this nasal secretion is “migrated” onto the test strip. This process is similar to that of pregnancy tests.
“It will be picked up by this test strip and it will give a colorimetric reaction,” continues Dr. Fafard. If there is no infection, the products will continue to migrate and will be picked up further on the control strip, to show that the test still worked. “
A simple, but fallible method
Unlike laboratory tests, where a rod is inserted very deep into the nose, patients must insert their swab up to the “middle turbinate”.
“You then have to make turns, pinch the nostril, everything to make sure to collect the maximum possible secretion”, advises Dr. Fafard.
Despite these precautions, the test may produce false results.
In 1% of cases, the test will give a false positive result, that is, the person is not infected, even if the test indicates it.
As for “false negatives”, the tests fail to detect the disease in 15% to 25% of the cases of infection with symptoms. “For the asymptomatic, we miss about half of the cases, if not a little more,” says Dr. Fafard. It is then advisable to repeat the test to ensure the validity of the result.
Caution is required with these tests, continues the specialist. “Before going to see someone who is extremely vulnerable, you shouldn’t just rely on a negative test. It would also be necessary to maintain precautions with the mask, distancing, etc. “
This text is taken from our newsletter “The Coronavirus Mail” of December 20, 2021. To subscribe click here.