The federal government first sent out free at-home COVID test kits a few months ago and are now sending a new round. As cases tick up, many of us may be reaching for a leftover kit we have lying around. But not so fast — make sure you check the expiration date on that kit before you use it, says Jared Auclair, director of the Biopharmaceutical Analysis Training Lab at Northeastern University, who joined GBH Morning Edition host Paris Alston to answer questions about COVID tests. This transcript has been edited for clarity and length.

Paris Alston: The producers here at Morning Edition and I were talking about how sometimes you might have some cold medicine sitting around that you might reach for in a pinch past the expiration date. But can you do that with one of these at-home COVID tests?

Jared Auclair: Great question, and let me circle back to the first part first. If you have expired medicine at home, please throw it away. We go through extensive testing of those medications to understand how long they're stable and useful for. And most people, including myself, used to think, "oh, it expired, it's just going to be less effective." Well, that's partly true. But also, the breakdown of those molecules can lead to all kinds of chemicals that you don't want in your body, including things that cause cancer. So throw it away and get new ones.

And the same thing with COVID tests — they expire. Now, they're not going to hurt you if you take an expired test, but an expired test will be less effective. You have a higher chance of seeing a false negative if you use an expired test. Why? Because the chemicals on that test, the antibodies that we're using to detect other chemicals, break down and don't work.

Alston: So help us understand why they don't work. What's the science behind the at-home tests and how does that break down over time?

Auclair: There are several different types of at-home tests, but most of the tests people have these days, sent from the government or from schools, are antigen tests. So what happens is, you take a sample from yourself. You put it on the cartridge, it absorbs into the cartridge and interacts with antibodies or protein molecules. Antibodies are chemicals that are proteins and they can break down. So that antibody has a little chemical on it — that's what we detect, what color metric causes the color that you see on the test. [If] the antibody breaks down or that molecule breaks down on that antibody, the detector — you won't be able to see. So you won't know if you're positive or not. You might get a false negative.

You can think about it this way — antibodies are proteins and we eat protein, and you wouldn't want to eat expired meat because it'll make you sick. And it looks gross. It's the same sort of concept — your meat will go bad because the protein is degrading. The same thing happens with these tests and those antibodies won't be able to detect the presence of SARS-CoV-2 because they, for example, might not be able to bind or stick to the virus, which will allow us to know that it's there.

Alston: So how do the manufacturers decide when a test should expire?

Auclair: It's a long process of determining how stable those molecules are over time, using a bunch of science — we do a bunch of experiments to prove how long those tests are good for. And the same thing with medicines. And that's why I really encourage people to take expiration dates seriously. And I'm just as guilty as everybody else.

"They're not going to hurt you if you take an expired test, but an expired test will be less effective."
-Jared Auclair, Northeastern University

Alston: I saw that earlier this year the FDA actually extended the recommended shelf life of a couple different tests. So, for instance, with the BinaxNOW test kits, those were extended three months based on data from drug maker Abbott. So is it worth hanging on to those tests in case we do get more data and the shelf life can be extended in real time?

Auclair: Generally speaking, I would recommend not doing that. But in the case of COVID in a pandemic, and to your point there being not as much data available as one would like — I think it can't hurt to save those tests just in case the expiration date extends. However, I say that it's not a bad idea with the sole thinking that in the pandemic, if there's a continuous spike and tests become harder to get and you throw them all away, and the expiration date was extended — I would be kicking myself saying, why did I throw those out?

But — if you do that, I would encourage you to check the FDA's website to see if the expiration date has been extended and to look very closely to make sure that the test the FDA is talking about is the one that you're holding in your hands, because there are many tests on the market and so many of them look so similar. Every expiration date is very specific to the individual tests that the manufacturer is producing.

Alston: And lastly, Jared, say that you maybe have a test that got left in the car on a hot day. Should you be using that or should you throw it out?

Auclair: That's a great question. We do all kinds of stability tests for drugs and for diagnostics like these tests in different conditions. The [COVID test] box has some protection, but if you leave the test in your car and it's like 900 degrees in there, like my car is when I open the door — throw it away. No matter what the expiration date is, if it's been in your car for, I would say, longer than five minutes even, there's a pretty high probability that a lot of those protein molecules will break down. We know that proteins break down in heat.