Thank you, that is super interesting and informative.
I am wondering if what we’re seeing here is cross-reactivity. (I’m not sure if that’s the right term for it, but: repeatable false positives from the test reacting to some antigen that is “close enough” to the covid antigen it’s looking for.) I recall seeing a table of things that one of the tests was checked for cross-reactivity against—they generally aren’t good at distinguishing SARS from covid, they mostly don’t react to other stuff, but the rate is not zero. (It’s possible the thing I am thinking of was an isothermal NAAT test, not antigen.)
Looking for info about this kind of thing with antigen tests, I eventually found this:
They have three case studies of antigen test false-positives, in Japan, in children infected with Human Rhinovirus A, and one of them remained positive on a test from a different production batch (although not a different brand/test.) And they mention that other countries have reported cross-reactivity with “other coronaviruses, influenza virus, and Mycoplasma pneumoniae”.
If it eventually went away and didn’t come back, I think after reading this I’m going to put my money on “cross-reactivity to another asymptomatic viral infection.” If it didn’t go away, and you still test positive on further tests, that becomes even more interesting and maybe you could get someone to write a case study about it in exchange for figuring out what the heck it is.
Thank you, this is quite interesting! I did consider the possibility of some other virus being the cause of the positive tests, but it strikes me as somewhat odd to have 32 days in between positive tests. In any case, the explanation to this situation is probably something very unlikely, so this could be it. This situation is recent (my last positive antigen test was a few days ago), so I might wait for a few more weeks and do some tests again to see what happens.
[Update] Over the course of the past month (starting ~7 weeks after my last false positive), I’ve taken 8 rapid tests, 4 of which from Acon (the brand that is most correlated with my false positives). All of them were negative, and one of the Acon ones had a very faint test line (which I still interpret as being negative, though I didn’t confirm with a PCR). The only thing I noticed I did differently this time around was that I took most of these tests (6 of them) past 11am, and not right after I woke up, as I used to do before, and the one with a faint test line was among the two I took within 2 hours of waking up. I never eat or drink anything 30 minutes before testing, as the instructions recommend. My impression is that this is reasonably good evidence that whatever was triggering the false positives is now mostly gone, as per in gwillen’s comment about cross-reactivity. Besides the observation above I have no reason to think that the time of the day when I take it should make much of a difference to the test results; I might test that out in the coming months. As before, I’m happy to read any other possible explanations for this, and might also test them out in the future.
Thank you, that is super interesting and informative.
I am wondering if what we’re seeing here is cross-reactivity. (I’m not sure if that’s the right term for it, but: repeatable false positives from the test reacting to some antigen that is “close enough” to the covid antigen it’s looking for.) I recall seeing a table of things that one of the tests was checked for cross-reactivity against—they generally aren’t good at distinguishing SARS from covid, they mostly don’t react to other stuff, but the rate is not zero. (It’s possible the thing I am thinking of was an isothermal NAAT test, not antigen.)
Looking for info about this kind of thing with antigen tests, I eventually found this:
https://onlinelibrary.wiley.com/doi/10.1111/ped.14582
They have three case studies of antigen test false-positives, in Japan, in children infected with Human Rhinovirus A, and one of them remained positive on a test from a different production batch (although not a different brand/test.) And they mention that other countries have reported cross-reactivity with “other coronaviruses, influenza virus, and Mycoplasma pneumoniae”.
If it eventually went away and didn’t come back, I think after reading this I’m going to put my money on “cross-reactivity to another asymptomatic viral infection.” If it didn’t go away, and you still test positive on further tests, that becomes even more interesting and maybe you could get someone to write a case study about it in exchange for figuring out what the heck it is.
Thank you, this is quite interesting! I did consider the possibility of some other virus being the cause of the positive tests, but it strikes me as somewhat odd to have 32 days in between positive tests. In any case, the explanation to this situation is probably something very unlikely, so this could be it. This situation is recent (my last positive antigen test was a few days ago), so I might wait for a few more weeks and do some tests again to see what happens.
Would love an update if you do!
[Update]
Over the course of the past month (starting ~7 weeks after my last false positive), I’ve taken 8 rapid tests, 4 of which from Acon (the brand that is most correlated with my false positives). All of them were negative, and one of the Acon ones had a very faint test line (which I still interpret as being negative, though I didn’t confirm with a PCR).
The only thing I noticed I did differently this time around was that I took most of these tests (6 of them) past 11am, and not right after I woke up, as I used to do before, and the one with a faint test line was among the two I took within 2 hours of waking up. I never eat or drink anything 30 minutes before testing, as the instructions recommend.
My impression is that this is reasonably good evidence that whatever was triggering the false positives is now mostly gone, as per in gwillen’s comment about cross-reactivity. Besides the observation above I have no reason to think that the time of the day when I take it should make much of a difference to the test results; I might test that out in the coming months. As before, I’m happy to read any other possible explanations for this, and might also test them out in the future.
Thanks for the update! This is really interesting to follow along with.