In the time between drafting this post and publishing it, I’ve now tested negative for the first time on an unexpired test 🎉
I have covid this week and I’ve been (purposely) using expired antigen tests to track it. The way antigen tests age is a gradual decrease in sensitivity¹. A strong signal may become weak; a weak signal may drop out entirely.
I already knew I had covid; I had a known exposure and symptoms, then tested positive two days later. I had a stockpile of expired Quidel QuickVue tests², so I figured it would make an efficient use of resources to use them to track my infection. The idea is that, with reduced sensitivity, I could still largely rely on true positives. If the test line showed up, I almost certainly still had an active covid infection.
On the other hand, if the test was negative, reduced sensitivity meant it would not be a reliable indicator I was done with my infection. I would need to follow up with a more sensitive test to confirm. But I could at least conserve my unexpired tests by using them only when needed.
On approximately day five of my infection (seven days after exposure, five days after my first definitive symptom, four days after first positive test) I was feeling a lot better. My symptoms had declined to an occasional cough and sniffle. Using one of my expired tests I got a very faint positive.
I knew in theory reduced sensitivity might mislead me here, but never having seen it demonstrated, I was hopeful perhaps it wasn’t a large factor and I would be over my infection soon.
On the fifth day I used my final expired Quidel test and came up negative.
Alas, when I tried to confirm with an unexpired BinaxNOW³ I got a clear, “nope, your infection is still kicking.”
So, despite feeling mostly healthy, I’m still in isolation. At least I’ve done a cool empirical demonstration of the way antigen tests degrade. Here’s hoping it won’t be too much longer.
(Obligatory caveat: I’m not a scientist and none of this is medical advice or even a sound scientific demonstration. But it is kinda neat, no?)
At least this is what I was told on Twitter once by someone who worked as an antigen test developer. ↩︎
Truly expired, even accounting for the extensions. If you have tests you think are expired, be sure to check them against the FDA list! Many tests have had their expiration dates extended by even a year. My tests expired on June 29, 2023, including their extended life. ↩︎
This particular demonstration would be more effective if I had used another Quidel test, but I didn’t think about it in time and I didn’t really feel like blowing a third test in a single day just for the sake of science – tempting as it is. I somewhat suspect BinaxNOW tests are more sensitive than their competitors even regardless of expiration date. ↩︎