Over the last few weeks, rapid tests have detected many COVID cases, but often not as early as we’d like them to.
With a bit of biohacking, and careful interpretation of test results, rapid tests can still be a valuable tool, despite their imperfections.
THE GOOD NEWS
Most rapid tests (aka lateral flow assays) can detect the Omicron variant – when there’s plenty of virus on the swab. We know this from lab studies and from many success stories, including families like mine. We caught an asymptomatic infection in my 10 year old son only 48 hours after exposure. This rapid test spared his friends and family from getting infected, as we were able to quickly isolate him from others.
The other piece of good news is rather twisted. Now that Omicron is everywhere, false positive tests are less of an issue for rapid tests. In other words, if you get a positive test, it is very likely to be a true positive.
THE BAD NEWS
In many cases, rapid tests have failed to catch Omicron in the early stages of infection. Stories are piling up from people with symptoms whose rapid tests were negative when symptoms began, but positive for COVID-19 a few days later. This is a change from pre-Omicron, when rapid test results typically aligned well with symptoms and contagiousness.
Why the *potential* disconnect between symptoms and rapid test positivity (potential, because we’re looking at anecdotes not studies)? This gap could be due to a change in disease biology – the nose may be late to the party with Omicron.
Some scientists have proposed that the Omicron variant may replicate sooner, and better, in some tissues like the throat and bronchus, than the nose. Others have proposed that, in vaccinated people, the virus may trigger an immune response that kicks off symptoms before there’s enough virus in the nose to detect with antigen tests.
There may also be subtle changes to analytical (lab) performance for some brands, though initial studies are reassuring.
CAN WE CATCH MORE CASES?
We may be able to more reliably catch Omicron earlier by changing where we look. There is a lot of chatter on Twitter about swabbing the throat AND the nose (yes, often with the same swab!) to better detect Omicron. In fact, this is standard practice for some rapid (lateral flow) tests in the UK and is supported by some pre-Omicron studies.
There is also emerging evidence that saliva or sputum may give an earlier signal than nasal samples. While PCR tests using saliva (or sputum) are fairly common, rapid tests for these specimens are currently only designed for professional use – though that could change soon.
As you can see, rapid testing results are shaped not just by the brand of the test, but also by other factors such as the sample site, test timing, sample processing and more (see figure).
⚠️ All COVID tests are very sensitive to test conditions and can be thrown off by pH, salts, temperature, etc. Whenever you veer from a test protocol, you risk messing with test performance. To reduce this risk, most throat and saliva tests recommend not eating or drinking 30 min before testing.
👉🏽Assume that any positive rapid test (even a faint line) is a COVID-19 case.
👉🏽Know that it takes several negative tests over a period of days to rule out COVID with confidence. One negative rapid test is not enough, especially early in infection. While in limbo, act as if you’re positive – isolating from others at home and beyond.
👉🏽Consider doing two tests, one swabbing nose only (per protocol), and another swabbing both throat and nose as a *potential* signal boost – if you are fortunate to have tests available. Follow directions in the link below.
On a personal note, I want to share a few big feelings about this topic.
💔 It’s heartbreaking that so many people got COVID over the holidays despite taking precautions, including rapid tests.
😡It’s infuriating that rapid tests are still unavailable to most people, both in terms of cost and supply. Once again, those with the fewest resources are suffering the most.
🙏🏽 I’m grateful to live in an amazing era for science and medicine. Rapid tests may not be perfect, but they have nonetheless prevented many COVID cases and saved many lives.
Stay tuned as we learn more about how to make the most of rapid tests and how each brand performs with Omicron. We would love to hear your Omicron rapid testing story.
Nerdy Girl Chana Fueled by Science
Analytical sensitivity of seven SARS-CoV-2 antigen-detecting rapid tests for Omicron variant (pre-print, not peer reviewed)
Saliva swabs are the preferred sample for Omicron detection (pre-print, not peer reviewed)