Your questions about home COVID-19 tests, answered

With cases rising, many people are scrambling for tests and information on how and when to use them

At home COVID test. (Copyright 2021 by WKMG ClickOrlando)

The omicron variant is here, and with it a huge demand for COVID-19 tests. Home tests are appealing because they allow you to avoid going to a testing center (with its long lines) and they produce results in just a few minutes.

“There’s a tremendous amount of enthusiasm and desire for these tests,” says Larissa May, MD, a professor of emergency medicine at UC Davis in California. But questions abound about how accurate they are, and how and when they’re most useful.

[TRENDING: CDC is investigating more than 80 cruise ships for COVID-19 cases | Can drivers legally move into an intersection while they wait to make a turn? | Become a News 6 Insider (it’s free!)]

What Kind of COVID-19 Tests Can I Take at Home?

Right now there are three kinds of at-home COVID-19 tests. The ones you’re probably most familiar with are rapid antigen tests, like BinaxNow, which you can pick up over the counter at a pharmacy (at least when supplies are available) and get results in under an hour.

There are also some options for at-home molecular tests (which are generally more accurate than antigen tests; see below for more information). PCR tests are available—LabCorp’s Pixel is an example—but they require mailing your sample to a lab and waiting for a result. Rapid at-home molecular tests are also available, from companies like Cue and Lucira, but they’re pricey and thus not widely accessible to most people.

How Do Home Antigen Tests Work?

Antigen tests detect proteins on the surface of the SARS-CoV-2 virus that provoke your immune system to respond. Because they react only to proteins actually contained in the test sample, they can return a false negative result when virus levels are low, as is generally the case, for example, when someone is infected but hasn’t yet started showing symptoms.

That’s in contrast to the molecular-based tests, including PCR tests, which detect the genetic material of a virus. These tests work by amplifying any existing genetic material present in a sample, by as much as a billionfold, according to Nathaniel Hafer, PhD, director of operations for the UMass Center for Clinical & Translational Science and an assistant professor in the program in molecular medicine at the UMass Medical School in Worcester, Mass. “This allows these tests to detect extremely small amounts of genetic material in a sample,” he says. “This amplification step makes them highly accurate, whether or not a person has symptoms.”

Are Home Antigen Tests Accurate?

Yes. “At-home antigen tests are quite accurate, especially if you have symptoms,” Hafer says.

Antigen tests are more likely to return a false negative if you’re infected but you don’t yet have symptoms. That’s because they might not be able to pick up the low viral load of an early or waning COVID-19 infection. But when you’re symptomatic, antigen tests may be a reasonable marker of whether someone is infectious, May says, at least at the time they take the test. Note, however, that groups such as the Infectious Diseases Society of America (IDSA) have so far held that neither rapid antigen tests nor molecular tests can be taken as definitive proof of a person’s infectiousness.

The likelihood of a false result on an antigen test also varies depending on the prevalence of COVID-19 in a community, a March 2021 Cochrane review found. When there’s little virus circulating in the area, a positive result on an antigen test is more likely to be a false positive, and you should confirm the result with a PCR test to avoid needlessly isolating yourself. However, when there’s a lot of COVID-19 circulating, as there is in most places in the U.S. right now, the researchers found that there’s a higher risk of a false negative result on an antigen test.

The different rapid antigen tests available have different levels of accuracy. The Medical Letter, a publication for physicians, offers this chart summarizing their accuracy (PDF) based on data from the test manufacturers. (The PPA, or positive percent agreement, and NPA, or negative percent agreement, indicate the rate at which the antigen test results agree with a positive and a negative PCR result.) Keep in mind, however, that some independent studies of test accuracy have found that the real-world accuracy may be lower than what manufacturers report in their data, at least when it comes to the ability of a test to return a true negative result in someone without symptoms.

How Much Do Home Antigen Tests Cost, and Where Can I Get Them?

Drugstores such as CVS, Walgreens, Walmart, and others sell OTC antigen tests. Prices for a package of two tests start around $14. Shortages are ongoing, so you may have to hunt around at chain and independent pharmacies to find them.

Some cities and states are distributing free at-home antigen tests, so check to see what may be available near you. Also, the federal government announced in December that it would soon be launching a site that will allow you to request free tests.

How Do I Use a Home Antigen Test?

The exact directions will vary by brand, so make sure to read them carefully before you begin. The Centers for Disease Control and Prevention recommends that you disinfect the surface on which you’ll be placing the components and wash your hands. Lay out all the materials in the test kit before you start the procedure, as if you’re practicing the chef’s technique of “mise en place” but for an infectious disease diagnosis instead of cooking.

The tests involve swabbing the inside of your nose—and it’s crucial to do this part correctly, so that you gather a good sample. Directions generally say to:

• Insert the soft end of the swab no more than ¾ inch into your nose.

• Firmly and slowly swirl the swab in a circle around the inside wall of one nostril at least four or five times, for a total of 15 seconds. Repeat this process using the same swab in the other nostril.

You then put the swab containing your sample into contact with some component of the test, such as into a tube of liquid that you’ve unsealed or against a card containing a test strip. Once you’ve finished all the steps, you’ll need to wait for a result. Be careful about keeping track of the time, according to what the test’s directions indicate. The results won’t be valid until a certain amount of time, but they’ll also become invalid if you wait too long. CareStart and QuickVue’s tests, for example, should be read 10 minutes after completing the procedure, but they become invalid after 15 minutes.

What Should I Do After a Positive or Negative Result?

If you test positive on an antigen test, given that there are high levels of virus circulating in most places in the U.S. right now, you can trust it even if you don’t have symptoms, so you should take the recommended isolation measures.

Guidance varies somewhat on how exactly to interpret a negative rapid antigen test result. If you test negative but have symptoms of COVID-19, it’s relatively simple: You should follow up with a PCR test to confirm, according to the CDC, the IDSA, and Wolters Kluwer UpToDate, a tool for doctors. Continue to isolate yourself as if you tested positive until you get the result.

If you test negative and don’t have symptoms, you can feel somewhat reassured that you’re less likely to be infectious at that moment in time, particularly if you’re vaccinated. (UpToDate and the CDC say people who are not fully vaccinated and have had a known or suspected COVID-19 exposure, no symptoms, and a negative rapid antigen test result should confirm with a PCR test or quarantine.) But it doesn’t necessarily mean that you aren’t carrying the virus and that you wouldn’t test positive if you repeated the test the next day.

“It doesn’t sort of give you a free pass to assume that there’s no way you could spread COVID,” Susan Butler-Wu, PhD, an associate professor of clinical pathology at the University of Southern California in Los Angeles, told the American Association for the Advancement of Science’s SciLine in an interview earlier this month. This helps to explain how people can contract COVID-19 at events such as concerts that require negative rapid tests right before entry.

Still, combined with other mitigation measures, such as masking and ventilation, getting a negative result on a rapid antigen test right before joining an event like a family gathering can help provide another layer of protection to attendees. You’ll want to take the test as close to the start of the event as possible—such as at the door. May says negative rapid antigen test results can also provide some added assurances in school settings where, again, other measures like masking and ventilation are also being used.

How Should Home Tests Be Stored?

Keeping tests at room temperature is fine. A study in the Journal of Clinical Virology found that storing kits above 86° F or below about 37° F decreased the accuracy of the tests, but most home environments are likely to be more temperate than either of those extremes.