I am that person. I’ll try to summarize my (bizarre) experience here, and I’d be happy to answer any further questions. I’m keen to see what people think could be happening, and would be willing to take some further rapid tests to test out good hypotheses (though this might take some time, as getting positive test results is very inconvenient).
I had my first positive antigen test after I had taken 5 negative antigen tests (daily) during the preceding week, and I had not taken any antigen tests before that. I took a second test after about one hour, which was also positive, and I took a PCR in less than 30 minutes after this second positive result. After receiving the result of the PCR on the following morning, and getting a negative result from another rapid test, I took a second PCR, which was also negative. I didn’t take rapid tests for a few days, but when I took my next one, 5 days after the first positive test, it was positive again, and the follow-up PCR test was again negative. I then started taking rapid tests less frequently, waiting for around a week between them, and have had two positives (followed by negative PCRs) and two negatives, in alternating weeks.
Some relevant information: no one around me has had covid during that time, and I haven’t had any symptoms. Someone watched me take a rapid test (which later turned out to be positive) and didn’t notice anything wrong with what I was doing. I took 13 rapid tests in total, 5 of which were positive, and 4 of these positive tests were from the same brand. 4 out of 5 of the tests of that brand that I have taken were positive. My first and last positive rapid test results are spread 32 days apart. I take my tests in the morning, after about 30 minutes from when I wake up, and before I eat or drink anything, though my second positive test was taken after I had breakfast.
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.
Wow, that is surprising, thanks for sharing. Am I reading correctly that you got no positive NAAT/PCR tests, and only got positives from antigen tests?
I took 13 rapid tests in total, 5 of which were positive, and 4 of these positive tests were from the same brand. 4 out of 5 of the tests of that brand that I have taken were positive.
Would you be up for sharing what brand that was?
I don’t yet know enough about what causes false positives and false negatives in either antigen tests or NAATs to speculate much, but I appreciate this datapoint! (Also, glad you’re feeling well and didn’t develop any symptoms)
Three of them (two positives, one negative) were from the same box, and the other two from different boxes. Other people at my house have taken tests from the same boxes, but no one else had positive results.
I am that person. I’ll try to summarize my (bizarre) experience here, and I’d be happy to answer any further questions. I’m keen to see what people think could be happening, and would be willing to take some further rapid tests to test out good hypotheses (though this might take some time, as getting positive test results is very inconvenient).
I had my first positive antigen test after I had taken 5 negative antigen tests (daily) during the preceding week, and I had not taken any antigen tests before that. I took a second test after about one hour, which was also positive, and I took a PCR in less than 30 minutes after this second positive result. After receiving the result of the PCR on the following morning, and getting a negative result from another rapid test, I took a second PCR, which was also negative. I didn’t take rapid tests for a few days, but when I took my next one, 5 days after the first positive test, it was positive again, and the follow-up PCR test was again negative. I then started taking rapid tests less frequently, waiting for around a week between them, and have had two positives (followed by negative PCRs) and two negatives, in alternating weeks.
Some relevant information: no one around me has had covid during that time, and I haven’t had any symptoms. Someone watched me take a rapid test (which later turned out to be positive) and didn’t notice anything wrong with what I was doing. I took 13 rapid tests in total, 5 of which were positive, and 4 of these positive tests were from the same brand. 4 out of 5 of the tests of that brand that I have taken were positive. My first and last positive rapid test results are spread 32 days apart. I take my tests in the morning, after about 30 minutes from when I wake up, and before I eat or drink anything, though my second positive test was taken after I had breakfast.
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.
Wow, that is surprising, thanks for sharing. Am I reading correctly that you got no positive NAAT/PCR tests, and only got positives from antigen tests?
Would you be up for sharing what brand that was?
I don’t yet know enough about what causes false positives and false negatives in either antigen tests or NAATs to speculate much, but I appreciate this datapoint! (Also, glad you’re feeling well and didn’t develop any symptoms)
You’re right, only the antigen tests were positive.
The brand was this one: https://www.aconlabs.com/sars-cov-2-antigen-rapid-test/.
Were the positive tests from the same batch/purchased all together?
Three of them (two positives, one negative) were from the same box, and the other two from different boxes. Other people at my house have taken tests from the same boxes, but no one else had positive results.