Rapid antigen tests (RATs) can detect some asymptomatic (symptom-free) COVID-19 cases, but they are far from perfect, and should be interpreted with caution.
TL:DR; A positive RAT is almost always right, but a negative result isn’t a guarantee that you are COVID-free. Best practice after a COVID exposure is to wait 5 days before testing, and re-test up to two times, a day or two apart if the last test was negative.
On a personal note, myself and several other Nerdy Girls have found rapid antigen tests to be extremely helpful in detecting asymptomatic cases following known exposures. For example, all three of my kids have had symptom-free infections that lit up brightly on RATs for several days. Because we learned we were infected by testing, we were able to ramp up precautions and avoid infecting family, friends, and the broader community. At the same time, many of us have also experienced confusing false negative rapid antigen tests. Whatâs going on?
The ability of RATs to detect asymptomatic (and symptomatic) COVID-19 cases can vary a lot depending on several factors. One of the biggest factors is timing – when you test in relation to when you were exposed, and how often you test. RATs are great at detecting the SARS-CoV-2 virus when there is *a lot* of viral protein present, but they are not so great at detecting small amounts of virus.
As much as we want to detect an infection ASAP, testing too early can lead to false negatives. It takes roughly 3-6 days after exposure for the virus to replicate enough for levels to be detectable with rapid antigen tests (note: timing is similar for cases with symptoms). Before and after this time, viral levels are lower, and less reliably detected by rapid tests (though often detectable by PCR). The sweet spot for RAT testing is around 4-5 days after exposure, when viral levels typically peak.
The Test US at Home study, which informed FDA testing guidance, gives a good sense of what to expect. They reported the following:
On average, when testing symptom-free cases between Day 0 and Day 6, RATs detected the virus 34% of the time with one test (note: Day 0 was the first positive PCR, so they exposure would be a few days earlier). By adding a second RAT two days after the first negative test, RATs caught 56% of cases. By adding a third test (another two days later), they detected 69% of asymptomatic cases.
The best results were on Day 4. At this time, a single RAT caught 75% of asymptomatic cases (again, Day 0 was the first positive PCR, a few days after exposure). This makes sense because levels of the virus typically peak around this time.
On Day 0, the day the virus was first detectable by PCR, a single RAT caught only 9% of asymptomatic cases. This tells us that when the virus is just beginning to replicate, there is rarely enough viral protein present for an antigen test to pick it up.
A recent review of many antigen testing studies reported similar numbers. Overall, they found that RATs detected 64% of symptom-free cases in people with a known COVID-19 exposure. When RATs were used more broadly, including for people without a known exposure, they only detected about 50% of symptom-free cases. It makes sense that test performance for RATs is worse when used for broad screening, because most people will not be testing during the âsweet spotâ if they arenât aware of their exposure.
Antigen test performance can also be affected by other factors, including sample type (where you swab), sample quality (how well you swab and process), test brand, and variant. These factors are beyond the scope of this post but weâre happy to elaborate. Read our posts below for more information and send us your questions!
The Bottom Line
The good news is that rapid antigen tests (RATs) can help catch sneaky, symptom-free COVID-19 cases – cases that can be contagious. The best practice is to test during the âsweet spotâ, roughly 5 days after an exposure. The bad news is that RATs still miss a lot of cases, so people may get a false sense of security, lessen their precautions, and transmit to others. To make the most of RATs, time your testing wisely, repeat 1-2 days later if possible, and bear in mind their limitations. A negative RAT is not a COVID-19 free guarantee.
Note: This Nerdy Girl has been fortunate to have access to unlimited free COVID-19 tests here in Vancouver, BC. We feel your frustration that widespread test access is not the norm – and that there hasnât been more education about how to use and interpret them.
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Related posts
How well do rapid antigen tests work and when is the best time to test?
Do rapid antigen tests detect the latest COVID-19 variants?
Help! Itâs Day 10 and my rapid antigen test is still positive. Am I contagious?
References
COVID-19 Testing: What You Need to Know (US CDC)
At-Home COVID-19 Antigen Tests-Take Steps to Reduce Your Risk of False Negative (FDA)
