COVID-19 rapid antigen tests detect the SARS-CoV-2 virus very well when used at the right time – when viral levels are high.
Repeat (serial) testing is a powerful way to catch more SARS-CoV-2 infections and reduce false negatives. In a recent study, COVID-19 rapid antigen tests detected the virus roughly 80% of the time when serial rapid antigen tests were used at 48 hour intervals.
Here are some practical tips for using rapid antigen tests wisely, along with the FDA’s updated advice and new data from a real-world study:
IF YOUR RAPID ANTIGEN TEST IS POSITIVE 🛑
➡️ FDA guidance: Assume you are infected following any positive rapid antigen test.
🔥Hot tip: A positive rapid antigen test is a good indicator that you could be contagious, regardless of symptoms. Always take a positive antigen test seriously.
📈Study results: Rapid antigen tests were negative 99.5% of the time in PCR negative participants.
IF YOU HAVE SYMPTOMS 🤒
➡️ FDA guidance: If negative, re-test in 48 hours (up to 2 tests total)
🔥Hot tip: For the best chance of detecting the virus, test 2-4 days after symptoms develop.
📈Study results: A single rapid antigen test detected 60% of symptomatic cases on Day 0, and 90-95% of symptomatic cases on Days 2-4 (Day 0 = first positive PCR). Overall, rapid antigen tests detected the virus 83% of the time with one test, and 93% of the time with two tests 48 hours apart (in symptomatic cases, Days 0-6).
IF YOU ARE EXPOSED BUT DON’T HAVE SYMPTOMS 🦠
➡️ FDA guidance: If negative, re-test in 48 hours (up to 3 tests total)
🔥Hot tip: For the best chance of detecting the virus, test 5 full days after exposure (exposure = Day 0; test on Day 6).
📈Study results: A single rapid antigen test detected only 9% of asymptomatic cases on Day 0 yet caught 75% of asymptomatic cases on Day 4 (Day 0 = first positive PCR). Overall, rapid antigen tests detected the virus 34% of the time with one test, 56% of the time with two tests, and 69% of the time with three tests 48 hours apart (in asymptomatic cases, Days 0-6).
RAPID ANTIGEN TEST ACCESS
Repeated rapid antigen testing is not currently realistic for most people due to limited access to affordable tests. However, limited free options are available in many places with a little sleuthing. We urge you to explore your options, ideally before you need them.
Residents of the United States can get eight free rapid antigen tests per household delivered by mail from the US government. Orders are easy to place online or by phone. See links below.
STUDY DETAILS & CAVEATS
Participants were enrolled Oct 2021 to Jan 2022 (mostly Delta and Omicron variants). Eligible participants did not have COVID-19 in the prior three months, were symptom-free in the two weeks prior to enrollment, and PCR negative in the first sample. They tested every 48 hours over two weeks with both antigen tests and PCR. Three brands of antigen test were used.
154 of 5,609 eligible participants tested positive for SARS-CoV-2 infection based on RT-PCR. Of these 154 PCR+ participants, 97 were asymptomatic and 57 had symptoms on the first day testing positive by PCR (Day 0)
⚠️ Rapid antigen test performance may vary depending on test brand and variant – study data was not broken down by variant or test brand.
⚠️ Rapid antigen testing every 48 hours can’t catch a small subset of infections called “singleton” infections – those that are PCR positive for only a brief time point. Nearly 1 in 5 (19/97) “asymptomatic” infections were “singletons” and never tested positive with rapid antigen tests. This pattern suggests vigorous clearing of the virus by the infected person -three cheers for the immune system.
⚠️The study is not yet peer-reviewed.
THE BOTTOM LINE
Rapid antigen tests aren’t perfect, but they’re still a very valuable tool – as long as we recognize their limitations and use them appropriately. We hope this post helps you figure out how to get the most bang for your buck.
👋 We’d love to hear your antigen testing stories!
Nerdy Girl Dr. Chana Davis @fueledbyscience
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