Free rapid Covid tests are coming back. How should we use them?

The fact is, we have not maximized their actual best use.

Image: D-Keine, Getty Images.

I was an early believer in the usefulness of at-home rapid Covid tests. Used correctly, they can decrease the spread of Covid-19. The Biden Administration announced last week that free tests will once again be available on the Covid.gov website.

Wait, what do rapid antigen tests tell us again?

A positive rapid antigen Covid test means you have Covid, and you’re likely to be contagious at the time of the test.

A negative test means different things at different times:

1. You have Covid, but it’s too soon for a rapid test to detect: It could be early, and you’re on the brink of having enough virus to turn positive on a rapid test.

2. You have Covid, but it’s too late for a rapid test to detect (i.e., you actually have had Covid for a week or two): You are likely no longer contagious. If you had taken tests a few days or a week prior, it would’ve been positive.

3. You don’t have Covid: If you test negative on a rapid every day for 6-7 days after a high-risk exposure (and never developed symptoms), it’s safe to assume you never got the virus (which is something a lab-based PCR test would definitively prove).

Everything would be perfect if people could remember the following about rapid antigen tests: Negative? You might have Covid but you’re not contagious now. Test again at least a couple more times over the coming days. Positive? You’re infected and contagious.

What should we be doing with the information rapid Covid tests provide?

How to act on the information at-home rapid Covid tests provide remains a source of great confusion. The CDC has an online tool that tells people what to do with their test results, depending on the circumstances. It’s basically a good resource, because it spits out answers based on the specific information you enter. You enter the date of a high-risk exposure or the first positive Covid test, and it tells you how long to stay home and how long to wear a mask. While I don’t agree with every answer it provides, it’s pretty good; if everyone who had a high-risk exposure or test-confirmed Covid followed the tool’s advice, Covid would spread a lot less.

A screenshot of the CDC Covid-19 isolation guidance page, with the link to the isolation timing tool circled in red.

Problem #1 with rapid tests in today’s world.

The problem with rapid tests is they often don’t mean what people think. As above, when done too soon after exposure, people think a negative test means they don’t have Covid. On the other hand, some people think that rapid tests miss cases altogether. That’s really only true if a test is done once (too early) and not repeated later. If you have Covid and you take rapid tests correctly, it’ll turn positive at some point. Meanwhile, some people don’t think a positive test means anything for those without symptoms. Not true; some asymptomatic people have high viral loads. Remember: people with high viral loads are at peak contagiousness and they reliably test positive on rapids. Symptoms may correlate to this, but not always.

In one dataset I recently analyzed, people with mild symptoms had lower average viral levels than those with moderate or severe symptoms. But, people with resolved symptoms had a higher average viral loads than those with mild symptoms. One patient whose symptoms resolved after 3 days had an extremely high viral load on day 7 (let alone they’d obviously been infected a 0-3 days before symptom onset). The CDC would have that highly contagious person running around town. I wouldn’t have.

All this says that binary yes/no answers on rapid antigen tests are safe to act on if you simply believe the result at the moment it was taken; don’t attempt to factor in symptoms or other factors (like vaccine status, or Paxlovid use). Some people might have very different combinations of symptoms and viral levels depending on their own immune status—a combination of their own immune system and prior exposures to the virus or vaccines.

Problem #2 with rapid tests in today’s world.

There’s actually another problem with rapid tests. Like PCR tests done in labs, rapids actually stay positive for longer than they should in many cases. PCR tests can stay positive for many weeks, well after the illness has genuinely resolved and the contagious window has closed. That’s why you don’t need to test negative on a PCR before ending isolation; we know it’ll be positive for far longer than a patient is contagious or ill. Meanwhile, rapid antigen tests often stay positive for 5-14 days. But after a 5-7 days, how contagious you are often drops by a huge amount—possibly 10-1000 fold! So, a person with a positive rapid on day 12 is likely much less contagious than they were on day 3-7.

In the early days of rapid tests, many people were willing and able to isolate for 5-14 days—until the rapids turned negative—because they did not want to spread Covid. Nowadays, even many well-meaning people don’t even do that. (The CDC does not require it—mainly because it is practicing “harm reduction.”)

So what’s an ethical person to do? That is—you may have Covid but you don't really want to isolate for 14 days (which is how long many people test positive on rapids). Meanwhile, you don’t want to be a sociopath, pretend nothing is wrong, go out and about after a couple of days and risk spreading Covid high-risk people.

Currently, the CDC’s guidance is the closest thing we have to threading that needle. Unfortunately, the guidance has two weaknesses. First, it takes symptoms into account, which it shouldn’t (on average, the guidance is right, but there are too many outliers). Second, its timeline is too simplistic. One person might have peak viral levels on day 3 after first testing positive, and another on day 7, regardless of symptoms. The CDC’s guidance allows some very viral people to go out. While they recommend masking at this point, that’s not enough during peak shedding (at other times, it probably is, with the right mask.) Theoretically, we can do better than this…but only theoretically, so far.

Moving beyond yes/no rapid tests.

There’s a trove of data on rapid antigen tests that you’ve likely never heard about. Turns out that these tests can do more than provide a yes/no answer on contagiousness. The brightness of the test line and how fast it appears likely correlate to how much virus you have and thus how contagious you are. There are probably a couple of days in which most of your viral particles are exhaled. We should be detecting those times and acting on them.

In the future, there may be rapid tests approved to give more than yes/no answers. Imagine a test that has two yes options. One says you’re positive. The other indicates whether your virus levels are extremely high. Because extremely high viral levels tend to only last 0-3 days, people could isolate only then, and safely wear an N95 mask (and avoid crowded spaces) for the next week. Isolating for the correct 2-3 days (when both “yes” lines were positive) instead of 5-14 days would decrease inconvenience substantially (and increase adherence), and—based on what we’ve learned about viral dynamics—still potentially decrease spread massively, perhaps by 90% or more, depending on the circumstances. Add good masking at the tail end, and air ventilation to public spaces, and we could decrease spread further.

The key is giving people information they are willing and able to act on. That’s the future we need to create. Currently, rapid Covid tests exist in an anti-Goldilocks situation; people who don’t want to be inconvenienced won’t take them at all, while the highly conscientious will but are then “punished” for being good citizens, consigned to longer isolation periods than necessary to achieve our goals.

The technology for this approach exists and is not costlier than current rapid tests. It’s just a matter of getting the FDA and the public to respond to the situation on the ground; That is, we should adjust what we are testing for and what to do about it. It’s 2023, not 2022, not 2020. Also, it’s not 2019 either.