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The U.S. Postal Service has been hard at work, delivering tens of millions of at-home COVID-19 tests to mailboxes across the country. Thus far, according to the White House, 60 million orders have been placed. Ultimately, the Biden administration vows to deliver a half-billion free of charge.

Sara Sawyer

“Ten dollars for an at-home diagnostic test that you can buy at Walgreens is really cheap, and free is even better,” said CU Boulder virologist Sara Sawyer, who has already stocked up on the government-issued tests for her lab and home. “People fail to realize how remarkable this effort is.”

With the tests’ arrival come a slew of questions: How well do they work? When’s the best time to take one? And what exactly should you do with the results? We asked Sawyer, professor of Molecular, Cellular and Developmental Biology (MCDB) and researcher at the BioFrontiers Institute, for her take.

How well do at-home tests work?

Before omicron I would have said they work perfectly well. They are not as sensitive as PCR (polymerase chain reaction) tests, but they still catch virtually everyone who has enough virus to be infectious to others. The people they miss are those who have such low viral loads that they aren’t dangerous to others anyway. But with omicron, the tests appear to be a little less sensitive.

How is the omicron variant different?

There’s nothing about this variant that makes it intrinsically hard to test for. It’s just that these at-home tests were designed to detect proteins based on previous variants. It’s like the tests were designed to recognize Navel oranges, and now we are asking them to recognize Mandarin oranges.

How does that impact how they work?

What I am hearing anecdotally from others in the field is that the tests don’t turn positive as early in infection as they did with previous variants. With previous variants, an at-home test may have picked up your infection one to two days before you had symptoms, now the at-home tests may only turn positive once symptoms start.

So are you more likely to test positive if you already have symptoms?

Yes. Your onset of symptoms corresponds pretty well to the time of very highest viral load in your body. Is it still worth taking it? Absolutely yes. The other option is to get a PCR test after which you may wait days to get the results. Most people in the field agree that a slightly less sensitive test where you get results faster is better than a more sensitive test in which you can be going to the grocery store for several days before you realize you have COVID. Ideally, if you think you have COVID, you should do the at-home test and do a PCR test so you can get both a quick answer and a definitive answer.

Some reports suggest throat swabs or saliva tests might be more sensitive than nasal swabs. Why might that be?

It’s very typical to get an itch in the back of your throat the day before you start showing symptoms. That’s because the virus tends to stick there, creating a little viral replication center soon after it comes in through your nose and mouth. Your saliva is a fluid that’s in touch with all of those places (think postnasal drip) and so we would expect to find virus there. Compare that with sticking something up your nose where, if it doesn’t touch a spot where viral replication is going on, you might not get a positive result. It’s something at-home testing companies are talking about. But for now, the Abbott Labs tests being distributed by the government are only FDA approved for nasal samples.

When should you test?

If you had an exposure yesterday, don’t waste your test by using it today. You are not going to test positive with any of these variants until three to five days after an exposure, so save your tests for those days. Also, if you are so sick that you are worried about your own physical state, that is not the time for an at-home test. Go to the doctor. The at-home tests are ideal for those cases where you’re like, “I kind of want to go see grandma but I have this tickle in my throat and I just don’t want to take any chances.’”

What if your test comes back negative?

No follow-up is really necessary unless you have strong reason to believe you do have COVID. If you had a known exposure to a person who had COVID and you have symptoms, there might be a reason not to trust the negative result that you got—particularly with omicron, as the tests do a little worse at detecting this variant. If you test too early or too late it may not detect it. Taking another at-home test a day or two later to verify would be reasonable.

What if it comes back positive?

A positive at-home test is a pretty definitive marker that you have COVID. False positives (the test saying you are infected when you are not) are very rare with COVID tests in general. In a perfect world, you should probably get a follow-up PCR test, but keep in mind that if you do that at a place like an urgent care or doctor’s office you are potentially exposing others. On the other hand, if you are worried for your health, you should always travel to a healthcare facility.

My at-home test is expired. Can I still use it?  

This is not recommended. If you have multiple tests at home, take a marker and write the expiration date on each box in large writing. Then, use your tests in the order in which they will expire.

Will distribution of free tests shorten the pandemic?

Yes, I believe it will. A lot of people will modulate their behavior based on being able to find out within 15 minutes in their own home that they have COVID. Parents are already using them to arrange play dates— testing all the kids in the morning and letting them get together without masks if they test negative. These at-home tests are not perfect, but they are pretty good and they are allowing people to get back some sense of normalcy that they have missed out on for two years.

For information on CU Boulder campus COVID-19 protocols and testing options, visit the Public Health Office page.