possible Flu B... or false positive?

[mindstalk]

Last night I had a bit of runny nose. This is something that happens on occasion, resolving overnight, and I live with a cat, to which I should be allergic. Still, I decided to test today, with combined Flu/Covid tests.

iHealth: bright red control line. Colorless moisture line at Flu B. Okay, that's odd. I've used these before and simply had clear results, control line and nothing else.

FlowFlex: bright red control line. Nothing otherwise. Great! Except, some time later, well past the 30 minute validity period, a faint pink line at Flu B. I have not used these tests before, though back in 2023 it's possible I kept getting FlowFlex false positive covids by checking too late.

So what to make of this? Neither test gives a clear positive result: one wasn't colored, the other showed nothing at 15 minutes. OTOH, two different tests showing a trace of Flu B is rather suggestive.

I tried searching on Flu B false positives... and actually got 4 hits:

2012 cluster of false positives on RAT https://pmc.ncbi.nlm.nih.gov/articles/PMC3421782/

2019 Texas A and B https://pmc.ncbi.nlm.nih.gov/articles/PMC8014320/

2019 France https://pubmed.ncbi.nlm.nih.gov/30971360/

2014 hospital https://academic.oup.com/cid/article-abstract/59/4/604/2895367?redirectedFrom=fulltext&login=false

So it's something that happens, maybe diffusion errors (which might be related to "moisture line" and "faint line after waiting excessively long.) As one of them said: when prevalence is low, the tests aren't very useful.

Is prevalence low? Flu is brisk at the moment, though PA is "high" rather than "very high". And A outnumbers B by like 30 to 1. And I wear a mask most of the time I'm around others, and I'm not around others that much, so my personal 'prevalence' should be low. But not zero: I do live with someone who's cautious but takes his own non-identical risks, and we have a contractor in (whom I avoided, but maybe the air didn't clear as I'd hoped.) And I don't really have symptoms other than a short-term sniffle.

So, very very minor infection, or false positive (or false quasi-positive, given that neither actually gave a fully valid positive, just deviation from a clear negative)? I dunno... Test again tomorrow or so, I guess. More accurate would be to go get a PCR test, but I don't have a doctor, I'm not sure what my insurance situation is, and going to a clinic full of sick people when I don't have symptoms, just for a flu resolution, doesn't seem worth the risk...