COVID rapid antigen tests and infectivity
August 2, 2024 11:43 AM
I could use some help interpreting inconsistent COVID rapid antigen test results and determining what my behavior should be in response.
Background
To avoid repetition of stuff I've already researched, I understand the difference between rapid antigen tests and molecular (typically PCR) tests. My summary would be that the former are less sensitive¹ since they lack the amplification step, but highly specific, so false negatives are common, especially early in an infection, but false negatives are quite rare.
I was previously bitten by this back in 2022, getting my first COVID infection from someone who had negative rapid tests for 3 days in a row and then later tested positive. But it's also the case that because PCR testing is so sensitive and detects RNA fragments which may not be viable, it's a good leading indicator whereas rapid tests are better for tracking the waning viral load during recovery. I have personally experienced the gradually fading positive test line over days of testing.
If any of the above is wrong, I'd be very interested to know, but include it so save respondents having to explain otherwise. Moving on…
The Current Conundrum
About a week ago (last Thursday) I started feeling some mild sinus congestion which got pretty bad over the weekend, and then was pretty much resolved by Monday. I had no other symptoms, nor notable exposures so I was starting to wonder whether I might be developing seasonal allergies. But to be on the safe side, I took a rapid test on Tuesday morning at it was clearly positive, so despite feeling totally symptom-free at that point, I started isolating.
To check on my recovery, I tested again ~2 days later, expecting to see a somewhat faded line, but to my surprise it was totally negative. While I wouldn't be surprised by false negatives on Ag tests in the early stages, I didn't expect it during recovery, so I doubted whether the test was accurate. I picked up another brand to confirm and got a very faint positive.
At this point, I'm fairly confused and am trying to answer two questions:
1. When should I go back to regular activities?
2. What should my testing regimen be?
Clearly, what I'd really like to know is whether I'm infectious to others, but as far as I know, that's not something any test can answer since people will shed differently and others may be more or less susceptible to infection (plus environmental factors, PPE, yada yada yada). What's not clear to me is whether we should expect the results across different rapid tests to be consistent either within the same brand/lot or across different brands.
The first two (clearly positive, clearly negative) tests I took were FlowFlex COVID-19 Antigen Home Test, of which I have 2 more. The third test (faintly positive) I took was CVS Health At Home COVID-19 Test Kit, of which I have 3 more. Since tests aren't super cheap anymore and my insurance doesn't reimburse (I checked), I want to make good use of what I have, but I also want to have the best understanding I can of my current status since there are activities this weekend I'd really like to participate in. I'm curious what knowledgeable people would suggest I do and why.
[1] In the technical sensitivity/specificity sense
Background
To avoid repetition of stuff I've already researched, I understand the difference between rapid antigen tests and molecular (typically PCR) tests. My summary would be that the former are less sensitive¹ since they lack the amplification step, but highly specific, so false negatives are common, especially early in an infection, but false negatives are quite rare.
I was previously bitten by this back in 2022, getting my first COVID infection from someone who had negative rapid tests for 3 days in a row and then later tested positive. But it's also the case that because PCR testing is so sensitive and detects RNA fragments which may not be viable, it's a good leading indicator whereas rapid tests are better for tracking the waning viral load during recovery. I have personally experienced the gradually fading positive test line over days of testing.
If any of the above is wrong, I'd be very interested to know, but include it so save respondents having to explain otherwise. Moving on…
The Current Conundrum
About a week ago (last Thursday) I started feeling some mild sinus congestion which got pretty bad over the weekend, and then was pretty much resolved by Monday. I had no other symptoms, nor notable exposures so I was starting to wonder whether I might be developing seasonal allergies. But to be on the safe side, I took a rapid test on Tuesday morning at it was clearly positive, so despite feeling totally symptom-free at that point, I started isolating.
To check on my recovery, I tested again ~2 days later, expecting to see a somewhat faded line, but to my surprise it was totally negative. While I wouldn't be surprised by false negatives on Ag tests in the early stages, I didn't expect it during recovery, so I doubted whether the test was accurate. I picked up another brand to confirm and got a very faint positive.
At this point, I'm fairly confused and am trying to answer two questions:
1. When should I go back to regular activities?
2. What should my testing regimen be?
Clearly, what I'd really like to know is whether I'm infectious to others, but as far as I know, that's not something any test can answer since people will shed differently and others may be more or less susceptible to infection (plus environmental factors, PPE, yada yada yada). What's not clear to me is whether we should expect the results across different rapid tests to be consistent either within the same brand/lot or across different brands.
The first two (clearly positive, clearly negative) tests I took were FlowFlex COVID-19 Antigen Home Test, of which I have 2 more. The third test (faintly positive) I took was CVS Health At Home COVID-19 Test Kit, of which I have 3 more. Since tests aren't super cheap anymore and my insurance doesn't reimburse (I checked), I want to make good use of what I have, but I also want to have the best understanding I can of my current status since there are activities this weekend I'd really like to participate in. I'm curious what knowledgeable people would suggest I do and why.
[1] In the technical sensitivity/specificity sense
Current CDC recommendations are to isolate until you have had 24 hours of no fever and symptoms clearly improving. After that you should still take extra precautions such as masking around others for the next five days.
This is much more lax than previous guidelines so you might want to be more careful if you are going to be around people who need to be particularly careful not to get COVID.
You can read here (section 3) for why CDC relaxed their guidelines. They say is a mix of better immunity and better treatment options. I think they are also trying to balance protection vs getting people who feel OK back to work. It also seems like the time to symptoms is much quicker these days than it was in the beginning so the infectious period is sooner and shorter than before but I don't have the date to back that up.
My personal preference is closer to the older CDC guidelines which would be to isolate for a minimum of five days from first symptom (or longer if you haven't had 24 hours of solid improvement and fever free) and then to mask scrupulously around others for another five days.
posted by metahawk at 12:46 PM on August 2
This is much more lax than previous guidelines so you might want to be more careful if you are going to be around people who need to be particularly careful not to get COVID.
You can read here (section 3) for why CDC relaxed their guidelines. They say is a mix of better immunity and better treatment options. I think they are also trying to balance protection vs getting people who feel OK back to work. It also seems like the time to symptoms is much quicker these days than it was in the beginning so the infectious period is sooner and shorter than before but I don't have the date to back that up.
My personal preference is closer to the older CDC guidelines which would be to isolate for a minimum of five days from first symptom (or longer if you haven't had 24 hours of solid improvement and fever free) and then to mask scrupulously around others for another five days.
posted by metahawk at 12:46 PM on August 2
Faintly positive is still positive. I think you are putting too much interpretive emphasis on the intensity of the line. In the end, it's intended to be a binary.
Now, as to whether you can attend those events, I believe the CDC would be fine with you interacting with the world at that point as long as you are symptom-free. I, however, personally would not feel great about deliberately interacting with elderly or immunocompromised people for nonessential purposes until I tested negative again.
posted by praemunire at 12:46 PM on August 2
Now, as to whether you can attend those events, I believe the CDC would be fine with you interacting with the world at that point as long as you are symptom-free. I, however, personally would not feel great about deliberately interacting with elderly or immunocompromised people for nonessential purposes until I tested negative again.
posted by praemunire at 12:46 PM on August 2
The CDC’s previous guidelines were that you can stop isolating after two serial negative tests 48 hours apart. I’m more comfortable following that guideline than the current one. A faint positive is a positive; you would need to test negative tomorrow, wait two days, then test negative again under these prior guidelines.
posted by capricorn at 12:57 PM on August 2
posted by capricorn at 12:57 PM on August 2
The current CDC guidelines aren't based on any scientific understanding of when people stop being infectious (often five to ten days). So for your purposes, you can ignore them.
Rapid tests are still the best way to determine if you are infectious. They are not perfect but they are the best tool we have.
But, a single rapid test is not as accurate as taking two tests. I've known folks who exited isolation after two negative tests 24 hours apart, which I think is also sensible.
The previous guidance to take two tests was also accompanied by a recommendation to continue masking for 10 days.
posted by mai at 1:18 PM on August 2
Rapid tests are still the best way to determine if you are infectious. They are not perfect but they are the best tool we have.
But, a single rapid test is not as accurate as taking two tests. I've known folks who exited isolation after two negative tests 24 hours apart, which I think is also sensible.
The previous guidance to take two tests was also accompanied by a recommendation to continue masking for 10 days.
posted by mai at 1:18 PM on August 2
Faintly positive is still positive. I think you are putting too much interpretive emphasis on the intensity of the line. In the end, it's intended to be a binary.But in this binary, what do 0 and 1 mean?
I understand the mechanics of the tests, and very much agree with mai's sentiment, "Rapid tests are still the best way to determine if you are infectious. They are not perfect but they are the best tool we have."
What I'm asking about in this question is whether the different tests are actually calibrated to anything consistent such that given the same level of antigen in the sample they'd consistently read positive or negative. I'm inclined to think that they different brands may just have different levels of sensitivity either because of different technology or manufacturing variability, but would like to have that confirmed (or refuted) by someone who understands more. Because in the end, I'd like to know what tests to take going forward and what to conclude from different results.
posted by Cogito at 2:04 PM on August 2
Reasons for a positive, a negative, and then a positive test could be due to anomalies with the tests, but could also be due to rebound or a slightly different swabbing technique.
posted by metasarah at 2:10 PM on August 2
posted by metasarah at 2:10 PM on August 2
Antigen tests are basically infectiousness tests. I would interpret the results you are getting to mean you are still a little bit (but not very) infectious, and maybe your swabbing or stirring wasn't quite as vigorous on the negative test. I would isolate until you get two negative tests 48 hours apart, maybe using different brands to test just to be safe.
posted by shadygrove at 2:46 PM on August 2
posted by shadygrove at 2:46 PM on August 2
So, after some digging on the kit instructions, the Flowflex limit of detection is 2.5 x 10^3 TCID50 per ml (50% tissue culture infectious dose, which is a measure of viral titer), and the CVS test limit of detection is 1.91 x 10^4 TCID50 per ml. This can't be a super meaningful difference otherwise the CVS test wouldn't be on the market, but technically the Flowflex test is slightly more sensitive because it can accurately detect lower levels of virus. I also don't know why the less sensitive test gave you a faint positive, maybe difference in sample collection?
All I would take from this is if you intend to do serial tests you should use the same brand of test.
posted by Shal at 3:19 PM on August 2
All I would take from this is if you intend to do serial tests you should use the same brand of test.
posted by Shal at 3:19 PM on August 2
Thank you, Shal! This is the kind of info I was hoping for.
I retested just now with the CVS one and it's hard to see in the image, but I think it has just the faintest possible positive line (zoomed in). I'm torn on whether to retest with the Flowflex or not. On one hand, if I got a faint positive, that would give me more confidence in the CVS result and I'd write off yesterday's totally negative Flowflex as a false negative, but if it were clearly negative again, I'd be just as confused since the Flowflex is notionally the more sensitive of the two tests.
I'm kind of leaning toward doing it since I have more of the Flowflex kits and I'd like to know whether to trust them or not, but maybe it's all just anecdotal at this point.
posted by Cogito at 3:38 PM on August 2
I also don't know why the less sensitive test gave you a faint positive, maybe difference in sample collection?I tried to do the swabbing the same way as much as possible, but the two kits are fundamentally different, so I think there's inherent variability.
I retested just now with the CVS one and it's hard to see in the image, but I think it has just the faintest possible positive line (zoomed in). I'm torn on whether to retest with the Flowflex or not. On one hand, if I got a faint positive, that would give me more confidence in the CVS result and I'd write off yesterday's totally negative Flowflex as a false negative, but if it were clearly negative again, I'd be just as confused since the Flowflex is notionally the more sensitive of the two tests.
I'm kind of leaning toward doing it since I have more of the Flowflex kits and I'd like to know whether to trust them or not, but maybe it's all just anecdotal at this point.
posted by Cogito at 3:38 PM on August 2
Just a reminder: 10% of covid-19 infections result in rebound even without paxlovid (where the rebound rate rises to 15%).
posted by srboisvert at 4:34 PM on August 2
posted by srboisvert at 4:34 PM on August 2
I think with such mild symptoms and faint line, you’re good to go. It could just be an artifact of how you’re shedding old cells out of your nose.
posted by haptic_avenger at 5:04 PM on August 2
posted by haptic_avenger at 5:04 PM on August 2
I retested just now with the CVS one and it's hard to see in the image, but I think it has just the faintest possible positive line
As someone who has squinted at a LOT of pregnancy tests, I would consider that test clearly, unambiguously positive. There is faint but visible line.
posted by maleficent at 7:30 PM on August 2
As someone who has squinted at a LOT of pregnancy tests, I would consider that test clearly, unambiguously positive. There is faint but visible line.
posted by maleficent at 7:30 PM on August 2
Also someone who has squinted at hundreds of dollars of pregnancy tests, in my (pre-kids!) days of anxiously trying to conceive. That is certainly a positive test.
As far as what that means for Covid: I don’t know. But that’s a positive test. For sure.
posted by samthemander at 12:41 AM on August 3
As far as what that means for Covid: I don’t know. But that’s a positive test. For sure.
posted by samthemander at 12:41 AM on August 3
Sharing this I guess just for the sake of anecdata, but I tested again today:
• CVS kit was more clearly positive today than yesterday
• Flowflex is as negative as can be
Uuuuuuugh.
posted by Cogito at 12:03 PM on August 3
• CVS kit was more clearly positive today than yesterday
• Flowflex is as negative as can be
Uuuuuuugh.
posted by Cogito at 12:03 PM on August 3
Sorry you're getting confusing results. As you can see in my most recent posts, I recently had stubbornly positive antigen tests for weeks, without ever having had symptoms except some fatigue -- never a fever or anything else. I searched a lot, and found nothing definitive about whether those results could mean I was still contagious after so long. I knew that the CDC's stance was that I should be back in the world as usual, but I wasn't comfortable with that.
On Day 24 of positive antigen tests, I finally went in for a PCR test. My understanding had been that people can test positive for much longer on those, and that testing positive as long as I had on antigen tests was relatively uncommon. But, I was negative on the PCR test. With advice here and from my doctor's office, that made me confident that I was no longer contagious.
I still antigen-tested periodically after that, just because in case I come down with it again at some point, I wanted to be able to know that any positive result wasn't still a holdover from this last time. I don't have the records in front of me right now, but it took well over a month for the antigen tests to be negative.
posted by daisyace at 6:28 PM on August 3
On Day 24 of positive antigen tests, I finally went in for a PCR test. My understanding had been that people can test positive for much longer on those, and that testing positive as long as I had on antigen tests was relatively uncommon. But, I was negative on the PCR test. With advice here and from my doctor's office, that made me confident that I was no longer contagious.
I still antigen-tested periodically after that, just because in case I come down with it again at some point, I wanted to be able to know that any positive result wasn't still a holdover from this last time. I don't have the records in front of me right now, but it took well over a month for the antigen tests to be negative.
posted by daisyace at 6:28 PM on August 3
You could consider shelling out for one of the home molecular tests like Lucira or Metrix. Of the two, I would suggest Lucira (lower cost than Metrix, also includes flu, and I got a bunch of Invalid results on Metrix despite carefully following the directions - though the company did offer to send me more to replace those).
posted by slidell at 5:24 PM on August 4
posted by slidell at 5:24 PM on August 4
There's no point in a molecular test as it's well established they can remain positive long past the point of infectiousness. I do not think that I've had 5 false positives with two different brands of Ag tests.
posted by Cogito at 10:28 AM on August 5
posted by Cogito at 10:28 AM on August 5
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Risk of infection probably depends on the context and the people involved. In our case we were both healthy 30/40 somethings who had been vaccinated and both previously had COVID. So YMMV but it doesn't sound super likely to this layperson that you're going to infect someone going about day to day life.
posted by knownfossils at 12:01 PM on August 2