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

Can heat damage at-home tests?

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.

I was exposed. When do I test?

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.”