COVID testing vs. screening
In my volunteer work with StaySafeScreen I’m hearing a lot of confusion between COVID testing and rapid antigen screening. PCR testing is the gold standard for diagnosing a COVID-19 infection, and rapid antigen screening is an tool for practically finding contagious people in the population. But when does it make sense to do one vs. the other? Here’s a handy rule of thumb:
Feeling fine? Screening time!
Not feeling your best? Isolate and test!
When you need to make a medical diagnosis of COVID-19, PCR is the right tool. It’s extremely important that we do everything possible not to miss a positive case. This means a PCR test usually involves a deep nasopharyngeal swab administered by a healthcare professional, and results will take a couple days to be generated in a lab. When you need to be sure, it’s worth the cost, wait and discomfort of getting a PCR test. For many other diseases, this is all we need. SARS, for example, was suppressed by isolating and testing just people with symptoms because symptoms manifest before patients became contagious.
Unfortunately, COVID-19 is different. At the very start of the pandemic the entire village of Vo, Italy were given PCR tests. Fully 70% of the infected people hadn’t had symptoms! It’s estimated that half of all COVID transmission occurs from someone without symptoms. Therefore testing just symptomatic people (who should be isolating anyway) is not highly effective at controlling the spread of the virus. The cost and logistics of PCR make it difficult to use for testing large populations on a regular basis. So in addition to the medical diagnosis tool of PCR, we need a public health tool to pragmatically reduce the risk of transmission from people without symptoms.
This is where regular, or “serial” screening with rapid antigen devices really shines. Antigen screens can be comfortably and conveniently self-administered daily in nearly any setting, with results available in about 15 minutes and with a cost of less than $7 CAD each. In England, most secondary school students and many essential workers self-administer an antigen screen at home twice a week. In one month alone this prevented over 25,000 contagious people from going to school or work where they likely would have infected others.
On the StaySafeScreen bus, we sometimes get people showing up with symptoms or with a known COVID contact asking for a rapid test. One person even told us that if we couldn’t give him a documented negative test result that day, he was going to lose his job. It’s heartbreaking, but we have to tell these people that we can’t help them. They have a high chance of having COVID-19 and so the consequences of a false negative from an antigen screen is too great. They must go get an official PCR test to be as confident as possible that they don’t have COVID-19.
Formal medical tools such as PCR tests are absolutely essential in this pandemic. To achieve a pragmatic combination of both openness and safety, it is essential that all business owners and individuals are also empowered with cheap and effective tools for uncovering themselves where this sneaky virus might be hiding. When an antigen screen indicates someone without symptoms may have the virus, then it’s time for them to isolate and get a PCR test.
Additional resources
- StaySafeScreen — our rapid antigen screening program in Waterloo region
- TEDx talk — Dr. Sameer Bakhda on rapid antigen screening.
- Michael Mina: “Covid-19 Testing: Where Science, Policy and Regulation Collide”
