Do Expired Covid Tests Work? What to Know During the Surge

Supported by

As Covid cases rise, at-home tests are a critical tool to keep yourself and others safe. Here are answers to a few common questions about when and how to use them.

By Dani Blum

Since the public health emergency expired, lab P.C.R. tests — the gold standard for detecting Covid-19 — have become more expensive and less accessible for many people. That largely leaves us with at-home rapid tests. As cases climb in some areas and reinfections crop up, here’s a refresher on how to use the tests most effectively.

Before you rip open a test that has been in your medicine cabinet since 2020, check the expiration date. If the test has expired, you can’t always trust the result.

“I don’t think it’s like having an old Ibuprofen or something,” said Dr. Marc Sala, co-director of the Northwestern Medicine Comprehensive Covid-19 Center. “I think you really need to take that seriously.”

That said, the Food and Drug Administration has extended the expiration dates for certain brands of tests. You can check a test by finding the lot number (typically found right by the expiration date) and cross-referencing it at this F.D.A. website.

Extreme heat can mess with tests. According to the F.D.A., at-home tests work best when you use them in an environment that’s between roughly 59 to 86 degrees Fahrenheit. If a test is delivered to you on a sweltering day, for example, the agency recommends bringing the package inside and waiting at least two hours before opening it. And always make sure the control line — which typically appears next to the “C” — shows up when you use a rapid test; otherwise, the test may be damaged or faulty.

If you have Covid symptoms and someone you had been spending time with is now positive, test immediately. Symptoms can include a runny or stuffy nose, fatigue, headaches, brain fog, sneezing or coughing, said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco.

If you were exposed to Covid and don’t have symptoms, the F.D.A. says to wait at least five days before testing because swabbing too early can give you a false negative. You should isolate as much as you can while waiting, and wear a mask when you are around other people.

If you have to spend time with someone who is immunocompromised or otherwise more vulnerable to the virus, like an older relative, consider testing before coming into contact with them, Dr. Sala said. While symptoms are a good indicator that you have Covid, there are still large swaths of people who get infected and never show any signs, he added.

The more immunity you’ve built up, the more subtle the symptoms often are, Dr. Chin-Hong said.

Symptoms also might come on sooner for people who have been vaccinated and previously infected, said Dr. Michael Mina, a former Harvard epidemiologist who is now the chief science officer for eMed, a telehealth company. “Especially people who have been recently boosted,” he said, “if they get infected, they might become symptomatic 24 hours post-exposure, 48 hours post-exposure — really fast.”

It’s also possible to first test positive after the five-day mark. There’s “wide variation” in when people test positive on a rapid test, Dr. Mina said. He estimated that roughly 30 percent of people test positive within three days after an exposure, another 30 to 50 percent within five days after, and about 20 percent test positive around day six or seven.

Rapid tests are most reliable when you have symptoms, said Dr. Prabhat Jha, a professor of global health at the University of Toronto. “If I’ve got an active symptom, I’ve got a runny nose, and then I test and it’s negative, I can be more sure that it actually is a true negative,” he said.

But one negative rapid test doesn’t mean you’re in the clear. You need to test again, typically 48 hours after the first test, depending on the instructions that come with your particular at-home test, said Dr. Heba Mostafa, director of the Molecular Virology Laboratory at the Johns Hopkins Hospital. If you do not have symptoms, the F.D.A. recommends testing at least three times after exposure, 48 hours apart. If you’re symptomatic, two tests may be sufficient.

Paxlovid rebound occurs in a minority of cases (it’s also possible to have a return of Covid symptoms even if you didn’t take an antiviral). In general, people who experience Paxlovid rebound will have a resurgence of symptoms between two and eight days after they recover, Dr. Mostafa said.

If you have symptoms within that time frame, you should test, Dr. Mina said, because rebound cases can be contagious. “If you are positive on a rapid test,” he said, “you should just assume you’re infectious. Period.”

Dani Blum is a reporter for Well. More about Dani Blum