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intrepidity

(7,734 posts) Fri Aug 23, 2024, 06:05 PM Aug 23

Re: expired Covid test kits and false positives

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.

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Re: expired Covid test kits and false positives (Original Post) intrepidity Aug 23 OP

Why do you think a false positive error is Voltaire2 Aug 23 #1

Because a false negative could happen because intrepidity Aug 23 #2

You're assuming degradation always results Voltaire2 Aug 23 #4

My married son and I both got sick within a day of each other and we'd been together two days before. chia Aug 23 #3

A false positive is very unlikely, especially if you have symptoms. subterranean Aug 23 #5

That's my thinking too intrepidity Aug 23 #8

The expiration dates of many tests drmeow Aug 23 #6

My doctor told me there are no false positives MotownPgh Aug 23 #7

Makes sense to me intrepidity Aug 23 #9

Voltaire2

(14,407 posts)

1. Why do you think a false positive error is

Reply to intrepidity (Original post)

Fri Aug 23, 2024, 06:09 PM

Aug 23

less likely than a false negative?

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

intrepidity

(7,734 posts)

2. Because a false negative could happen because

Reply to Voltaire2 (Reply #1)

Fri Aug 23, 2024, 06:18 PM

Aug 23

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

(14,407 posts)

4. You're assuming degradation always results

Reply to intrepidity (Reply #2)

Fri Aug 23, 2024, 06:53 PM

Aug 23

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

(2,321 posts)

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

Reply to intrepidity (Original post)

Fri Aug 23, 2024, 06:32 PM

Aug 23

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

(3,495 posts)

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

Reply to intrepidity (Original post)

Fri Aug 23, 2024, 07:11 PM

Aug 23

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.