With coronavirus cases on the rise, at-home testing remains a useful but imperfect way to mitigate risk.
I caught up with Katherine J. Wu, a staff writer who’s been covering this pandemic, to talk about what rapid-test results can—and can’t—tell us. Below you’ll find six takeaways from our conversation.
The conversation that follows has been edited and condensed for clarity.
1. With at-home tests, you trade accuracy for convenience.
“Doctors treating patients very often reach for those really sensitive PCR laboratory tests because they want to know beyond a shadow of a doubt whether the virus is present so the person can get the right treatment. Those tests are so sensitive that people can really trust when they’re negative. With the rapid tests, a negative is a lot less of a certain answer.”
2. At-home tests are most useful when a person has symptoms.
“A lot of the rapid tests we now use at home were initially designed to be taken as diagnostics for people who are feeling sick and want answers quickly. They then got the FDA greenlight to be used as screeners. Generally, right after your symptoms first appear is when they have been shown to be most accurate.”
3. They’re less accurate when used as screeners by people who feel fine.
“Things get a little tricky when you don’t have symptoms and you don’t have a known exposure—for example, if you’re hoping for a negative so that you can clear yourself to, say, go visit a grandparent.
“Rapid tests are not as good as lab tests when it comes to picking up on the virus when it’s present at really low levels. To get around that, experts say to test repeatedly. That way, if you miss the virus on Monday, maybe you’ll catch it Tuesday.”
4. At-home tests might be your only option.
“Let’s certainly not discount the value of rapid tests. With so many cases here and abroad, laboratory-testing infrastructure is going to be crushed. It may not be practical for people to seek out PCR testing as their No. 1 choice right now. If people are able to access rapid tests, they should test themselves daily at this point because we know results can change so quickly.”
5. All tests are mere snapshots of a moment in time.
“Tests cannot predict the future. The minute that swab goes into your nose—that is what the result is going to be giving you information on. We know this virus can replicate super quickly, especially if we’re talking about something like Omicron or Delta. A person can test negative in the morning and have a blazing positive result by the evening.”
6. Omicron is making things even trickier.
“We are still figuring out how quickly this variant spreads both within individual people and between different people. But based on what we know so far, it seems to be moving super fast, and there seems to be some indication that people can go from not contagious to quite contagious very quickly. And if that’s the case, then that means that negative test results actually expire sooner.”