Home tests still work to detect Covid-19, but here’s why your test may not pick


With Covid-19 cases up across the country, many people are once again relying on home tests to guide decisions about going to work and sending their kids to school and other activities.

A lot of those tests will come back negative, even when a person is 100% sure they have Covid-19. They have had a known exposure, for example, or telltale symptoms they recognize from a previous bout.

That has led to speculation on social media that perhaps rapid tests have lost their ability to detect some of the newer coronavirus variants.

“Each time a new variant comes around, I see this exact same conversation spark up on Twitter,” said Dr. Michael Mina, an immunologist and epidemiologist who is the chief science officer at the telehealth company eMed. Mina was an early proponent of selling rapid lateral flow tests to the public as a way to help people understand when they were contagious.

Mina says people who notice this aren’t crazy. There are reasons why your test might say you’re negative why you actually have Covid.

For one thing, people are using rapid tests differently than they used to. Our immunity factors, in, too. Nearly all of us have some kind of underlying immunity to the coronavirus now, and that affects the performance of the test, too.

But ultimately, the tests are still capable of picking up infections, said Todd Merchak, who co-leads the RADx program at the National Institutes of Health. The program, whose name is short for “rapid acceleration of diagnostics,” was created during the pandemic to quickly develop tests for the coronavirus.

“To date, the performance of currently marketed COVID-19 tests has not been adversely impacted by any new variants,” Merchak said in a statement.

RADx works with the US Food and Drug Administration to monitor new variants and continually assesses the performance of the Covid-19 tests sold in stores.

The reason Covid-19 tests continue to work when other tools don’t, such as the vaccines and monoclonal antibodies that have become ineffective over time, is that the vaccines and antibodies target the spike proteins that stick up from the round envelope of the virus. These spikes are under constant pressure from the environment to change, and they do.

Most rapid tests, on the other hand, target the nucleocapsid proteins, or N-protein, of the coronavirus. N-proteins don’t change as much as spike proteins do.

If the N-proteins do change, researchers are prepared.

In a large study published in September 2022 in the…

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