A rapid screening Regime is just as sensitive as PCR testing
Welcome to Plugging the Gap (my email newsletter about Covid-19 and its economics). In case you don’t know me, I’m an economist and professor at the University of Toronto. I have written lots of books including, most recently, on Covid-19. You can follow me on Twitter (@joshgans) or subscribe to this email newsletter here. (I am also part of the CDL Rapid Screening Consortium. The views expressed here are my own and should not be taken as representing organisations I work for.)
It was taken some time but studies have been coming in steadily that refute the often-made claim that rapid antigen screens are less sensitive than PCR test and so result in more false negatives. A new study has provided the clearest refutation of that contention yet by taking both PCR and rapid antigen tests from the same individual at the same time — this is one of the advantages of having not one but two nostrils.
RTqPCR tests will be more effective than rapid antigen tests at identifying infected individuals 60 prior to or early during the infectious period and thus for minimizing forward transmission 61 (provided results reporting is timely). All modalities, including rapid antigen tests, showed >94% 62 sensitivity to detect infection if used at least twice per week. Regular surveillance/screening 63 using rapid antigen tests 2-3 times per week can be an effective strategy to achieve high 64 sensitivity (>95%) for identifying infected individuals.
In other words, being “rapid” is important and so unless PCR tests are that, then the time where they have a benefit is not going to matter in practice. For instance, in the Ontario Science Advisory Table report on testing in LTC I referred to in my last post, PCR tests were conducted but most results came back in 2 days. That means that infected staff could have been interacting with residents during that time which the report rightly pointed out was problematic for the value of such tests.
This new paper suggests that beyond that initial period, PCR tests and rapid antigen tests have a similar value.
There is a difference. The rapid antigen tests peak at 96% sensitivity while the PCR tests get to 100%. Interestingly, the salvia PCR test also performs well.
But conduct the tests frequently and even those differences disappear into insignificance.
Given that it is easier to have more frequent rapid antigen screens than PCR tests, this suggests that a twice-weekly rapid antigen screen will perform better than a weekly PCR test. It will also be cheaper to do that as well!