Re: expired Covid test kits and false positives

[intrepidity]

Can anyone explain how or why an expired test kit might produce a false positive?

My opinion is that it won't, or shouldn't. A false negative, sure. But how would it produce a false positive?

I finally got sick after 5 years of not having one single cold or flu. I only had all the old covid test kits that I got via USPS, all of which had "expired" by at least a year. (Disclaimer: expiry dates on everything are very squishy to me so I usually take them with huge grains of salt.) So, I used a couple of the "expired" kits, one was the oldest and one the newest, and both showed a positive result. I did not think at the time to do a negative control (like maybe swab my dog's mouth?) but I did have my son and benefactor, who so graciously donated the virus to me, take the same tests, and his were also positive (surprise!!) but, since he was several days ahead of me symptom-wise, his test signal was noticibly fainter, which confirmed to me the validity of the tests.

A long way to say: don't discard those old "expired" kits, because they still may be useful.

But, if anyone can answer the original question. i'd appreciate it.

[Voltaire2]

1. Why do you think a false positive error is

less likely than a false negative?

Without more information I would start with either error is equally possible.

[intrepidity]

2. Because a false negative could happen because

the antibody/antigen on the card has degraded and thus will not report a signal.

But a false positive means that *something* is binding to something. The most likely things binding are the original intended targets.

I really should have done a negative control. I will repeat the tests again later today and report back.

[Voltaire2]

4. You're assuming degradation always results

In a lack of signal. You could be right. I offhand don’t know why degradation could also not cause an increase in signals.

I guess I view the tests as having a certain amount ‘noise’ in their results even when new, and that noise results in both false positives and false negatives. As the tests degrade over time, the noise level increases, the errors increase, but that applies to all errors.

[chia]

3. My married son and I both got sick within a day of each other and we'd been together two days before.

I took a Covid test and gave him my extra test. It was expired by about 9 months. My result was positive, his was negative.

Now I know that doesn't mean squat. It's not scientific. But either we both had Covid and one test worked and one didn't (same box) or we both didn't and one test worked and one didn't, or I had Covid and he didn't. There's no way to know!

But that's my experience, for what it's worth, which isn't much. I'm pretty sure I had Covid. It would be my second time.

[subterranean]

5. A false positive is very unlikely, especially if you have symptoms.

Like you, I also tested positive recently using a test that expired last year. My son had already tested positive several days earlier, and I had started to develop symptoms too, so I was pretty sure the result was correct.

A false positive doesn't really make sense, scientifically. An expired test might be less sensitive, so it would be more likely to give you a false negative, but there's no reason it would detect virus particles that aren't there.

[intrepidity]

8. That's my thinking too

Plus, the fact that my test band was really solid and his was less so, and that he had symptoms about a week before me.

[drmeow]

6. The expiration dates of many tests

were extended by the FDA, suggesting the tests last longer than the original expiration date. Were these expired based on the printed date or based on the new dates: https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/home-otc-covid-19-diagnostic-tests

[MotownPgh]

7. My doctor told me there are no false positives

[intrepidity]

9. Makes sense to me

Thanks for confirming

[truddy777]

10. Compliance

Last edited Mon Oct 28, 2024, 06:17 AM - Edit history (1)

They're useful if the lab that made them is compliant with HHS-OIG. Here's a short description of how it should be done: https://www.jdsupra.com/legalnews/5-tips-for-lab-compliance-in-2022-9912720/