How Well Do Rapid COVID Tests Detect Omicron and Its Subvariants?

While rapid antigen tests can still diagnose COVID-19, timing may be key for the most accurate result.

hand holding a negative coronavirus test
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Can you trust an at-home test to properly diagnose COVID-19, even with Omicron subvariants? Experts say yes—but note that antigen tests may be less sensitive to newer coronavirus strains.

According to the U.S. Food and Drug Administration, antigen tests—more commonly known as at-home tests or rapid tests—are generally less sensitive compared to molecular tests (PCR tests), especially regarding early infections. This sensitivity may decrease further, due to the genetic changes in the virus over time.

"Antigen tests are good home tests," Pedro Piedra, MD, a professor and pediatric infectious disease physician at the Baylor College of Medicine, told Health. "But understand that they are not as sensitive, and with Omicron variants they have become less sensitive."

However, when the tests are used correctly, it's still possible to be confident that you're getting a correct reading. Here's what we know so far about the accuracy of at-home tests—even when they're up against Omicron subvariants—and any tweaks you may want to make to your testing habits, to make sure you're getting the most accurate result.

Watch for Lowered Sensitivity in Early Days of Infection

Lower sensitivity doesn't necessarily mean that a test is completely inaccurate. In the case of at-home COVID tests, it may just mean you have to wait a few days.

Test accuracy is based on two factors: sensitivity, which refers to a test's ability to correctly identify patients with a disease; and specificity, which refers to a test's ability to correctly identify people without a disease.​​

At-home tests have a decreased sensitivity, which means they simply can't pick up on smaller amounts of virus present in the body, according to Bruce Tromberg, PhD, director of the National Institute of Biomedical Imaging and Bioengineering,

"If you have an early stage of your infection, with just a little bit of virus inside you, the antigen test isn't going to pick it up," Tromberg told Health. "You may get a negative [result] one day and then 12 hours later—because [of] the way that the virus is expanding so fast in your body—you could get a positive."

At-home antigen tests—like iHealth, Binax Now, and Flow Flex, for example—don't have great limits of detection, or the lowest amount of detectable virus in a given sample, Tromberg explained. Whereas a PCR test that you'd get in a lab could detect as little as 100 or 1,000 copies of the virus in one milliliter of fluid, antigen tests may not be able to detect the virus until that number is closer to 500,000 or a million copies, he said.

"Within 48 hours of your first onset of symptoms, the probability that an antigen test will test positive and agree with PCR is about 90%," Dr. Tromberg explained. "If you're very early and are like, 'Are these symptoms, I'm not sure,' then the antigen test just doesn't work there."

The fact that antigen tests miss positive infections early is true across the board for all variants, Tromberg argued. He runs the Rapid Acceleration of Diagnostics (RADx Tech) program with the National Institutes of Health (NIH), which has been responsible for helping fund, test, and commercialize COVID tests since the pandemic began.

Research funded by RADx in collaboration with the University of Massachusetts Chan Medical School found that there was actually very little difference between antigen test sensitivity for Omicron and Delta, though the study did not include data from newer Omicron subvariants. The largest difference in determining whether a test would pick up an infection, Tromberg said, was not the variant the person had, but rather how early into their infection they took their test.

Rapid tests seem to be less sensitive for newer COVID variants in general, a letter published in April by researchers from the Harvard T.H. Chan School of Public Health said. But some tests may be able to detect Omicron at lower virus levels even than Delta.

How to Ensure Your At-Home Test Result is Accurate

Because at-home tests don't have the capacity to detect lower levels of the virus, the CDC asks people to wait five days after an exposure before they take a test, or to start testing when they first experience symptoms. They warn, though, that testing negative while you still have symptoms could just mean you took a test before the virus was able to be detected.

Just to be safe, it's best to test multiple times, according to Dr. Piedra. Waiting a few days between tests will give you a better indication of whether you're actually negative or if the virus levels were just too low to be detected. Repeating tests could also lessen the possibility of user errors.

"We assume that one is taking the sample well, appropriately. And if you're self-collecting it and you're a little bit timid, you may not be getting a good sample," Dr. Piedra told Health. "And so that also has to be taken into account."

One other tool to ensure a more accurate test in the face of an evolving virus would be to look for multiple target at-home tests, the FDA recommended. Multiple target tests look for different parts of the coronavirus' genome or antigen spike proteins, which makes it more likely to detect the virus, even as parts of it mutate.

And, most obviously, the early detection issues with at-home antigen tests mean that it can be a good idea to get a laboratory PCR test to be safe.

Though there are a couple PCR tests that the FDA warned are likely unable to detect newer Omicron variants, the majority of PCR tests are more sensitive than at-home tests and you can be more confident that you're not getting a false negative.

This is especially important if you're at a higher risk of developing life-threatening complications from COVID, Dr. Piedra said.

"If you are using this to make a clinical decision and you are an at-risk individual, an older individual, an overweight individual, somebody that has chronic lung disease, heart disease, so forth, diabetes, and your test is negative but you're symptomatic, then I would say get a PCR test and talk to your doctor to see whether antiviral therapy is indicated or not," Dr. Piedra said. "Rapid antigen tests, because they're not as sensitive, you have to be careful [of] what a negative test result means."

Do At-Home Tests Need to Be Redesigned?

The NIH's RADx validation core team is continually evaluating tests as new variants crop up to make sure that they can still accurately detect the virus, Tromberg said.

But testing as we know it is changing, especially as more people forgo PCR tests entirely and just self-test at home. Those cases aren't necessarily reported to the government, and Tromberg fears the CDC is missing out on "a few 100 million tests per month" because of it. At-home tests also don't tell users which variant they have, which is information public health officials could use to better track the virus.

"It's not totally clear whether we need variant-specific at-home tests, when the at-home tests are really inexpensive, accessible, and they detect the variants," Tromberg said. "Do we need to know exactly what the variant is? That's still a big debate in the medical community."

RADx sees a future, though, where people can self-report COVID tests anonymously on an app from their phones, at-home tests have greater viral detection capabilities, and PCR home tests become more inexpensive and easily accessible, Tromberg said.

But until then, at-home tests seem to be able to detect Omicron and its many subvariants, so long as you avoid swabbing errors, wait a couple days after an exposure or symptom onset, and potentially test multiple times, if necessary.

"[At-home tests] are very very good if you use them where they're strong," Tromberg said. "And that's the key message."

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