Q: Do home Covid tests really expire? I ordered what I was allowed. Now I see they have expiration dates. Some have already expired. What to do? Keep and use, or throw them out?

It’s true that home Covid-19 tests are marked with expiration dates, but the actual expiration for a box of tests can be a moving target. Before throwing away a home test because you think it may have expired, do a little homework first.

Depending on which home test you buy, or receive for free from the government, you might see a range of expiration windows. One test might expire in six months, another in nine months, 11 months or even 15 months. The tests all use similar technology to detect antigens (pieces of viral proteins) from a swab sample — so why do the expiration dates stamped on the boxes vary so widely?

The answer has to do with the quirks of the regulatory process rather than any meaningful differences in the stability of the various tests, said Dr. Michael Mina, a well-known expert in home-test technology and chief science officer for eMed, a company that helps rapid test users get treatment from home.

When it comes to determining shelf life for any product it regulates, the Food and Drug Administration may allow a fast method or require a slower one. For some products, the agency will allow a manufacturer to rapidly simulate conditions — a process called “accelerated dating” — to show how long the item will last sitting in your medicine cabinet, said Mara Aspinall, an expert in biomedical diagnostics at Arizona State University who is also on the board of OraSure, which makes rapid Covid tests. This quick process can allow the manufacturer to determine in a matter of weeks whether a product has a shelf life of a few months or as long as a few years.

But in the case of the rapid home tests, the F.D.A. has asked for real-time data from manufacturers, which is a much longer process. Will a test that sits on the shelf still work after nine months? Twelve months? The only way to find out is to wait for the months to pass so the test makers can conduct stability studies to prove the tests still work over time.

As a result of this requirement, a home test might have a six-month expiration when it’s first authorized, but as more time passes the test maker collects more data and seeks an extension to the original shelf-life date. That means you might have a test at home that’s passed its expiration date, but if you call the company or dig through the F.D.A. authorization letters, you’ll find it has changed.

“When the test is new, it has a six-month expiration,” Dr. Mina said. “But once you get to six months, the F.D.A. may extend it. That’s been happening a lot, which is exceedingly confusing.”

Tests should be marked with a manufacturing date and an expiration date. An F.D.A. spokesman said that anyone with a question about an expiration date could go online to view the various regulatory documents that extend a test’s shelf life. There is a section for antigen tests here and molecular tests here. Then, you can do the math based on your test’s manufacturing date. For instance, in January, the F.D.A. extended the shelf life of the BinaxNOW test to 15 months, from 12 months, so many people can just add another three months to the expiration listed on their box. But sorting through F.D.A. authorization documents is a tedious and confusing process.

Health experts don’t want to advise consumers to use expired tests — but they don’t want you to throw away a perfectly good (and expensive) test either.

“Many people now have a small inventory of tests at home,” Ms. Aspinall said. “It would be a pity if somebody has symptoms, but they don’t use a test because it’s a few days out of date. If a test is days out of date, it’s highly likely it’s still effective. If it’s months out of date, it’s very important to check the website to see if the date was extended.”

A more practical approach is to make sure that your home test has been well cared for. Keep it at room temperature in dry conditions; don’t let it freeze or become exposed to heat, which can interfere with the accuracy of the test. If you’re planning to order more free government tests, do so now before the summer weather arrives so they’re not sitting for hours in a hot mail truck.

When you shop for a home test, check expiration dates in the store and find one that doesn’t expire for a while — just like you would do if you were buying milk. Pay attention to the expiration dates on the kits you have at home, and use the tests that expire sooner first.

And whether you’re using a new test or one that is a little past its expiration date, follow the instructions carefully — and make sure the control line shows up quickly, which is an indicator that the test is still working.

“The reality is that these tests are very, very stable,” Dr. Mina said. “My expectation is that most of them, if not all of them, eventually will have a two-year expiration date at least. If the control line is showing up and it’s within 18 to 24 months of the manufacture date, you should assume the test is working.”

Leave a Reply

Your email address will not be published. Required fields are marked *