I've been helping a school doing pooled testing (through Concentric). The protocol is to do pooled tests that are couriered to a lab for PCR, and then we follow up on any positive pools with individual antigen tests (BinaxNow) in the hopes of immediately figuring out who triggered the result.
We've had positive pools by PCR multiple times, only to have every single antigen test in that pool come back negative. In one case, one of the students became symptomatic two days later, so I assume she was brewing an infection and just didn't have enough virus in her nares to trigger a positive result. But on all the other ones - nada.
We've double checked the expiration etc on the Binax Now, tried using a different box of tests to immediately re-test students, triple-checked the protocol, had the nurses collect the samples rather than the students, meticulously rechecked every step of the process in place...but, nothing. Positive PCR, negative antigen.
I would expect this to happen sometimes just due to the difference in sensitivity between the two tests, as in the situation I described above, but I wouldn't expect it to happen almost every week. I wouldn't expect those all to be false positive PCRs. And the pools coming back positive do NOT include any kids with a COVID diagnosis within the last three months, so I don't think we're catching coronavirus RNA scraps from a previous infection.
It's really frustrating, and the school is questioning whether it's even worth the time/money to test. There's not a way for the BinaxNow swabs to reflex to PCR testing - they'd have to send all the kids home to PCR test in clinics or pharmacies. This makes sense conceptually and clinically, but is also likely to seriously erode parent support for the in-school testing.
Is this a common problem? Does someone else use a better protocol? How about you @JenTheSchoolRN?
