5 ms·

Antigen nose swabs are extremely unreliable, unfortunately. Barring public knowledge about this, the campaign could be ineffective at best or actively harm infe

Antigen nose swabs are extremely unreliable, unfortunately. Barring public knowledge about this, the campaign could be ineffective at best or actively harm infection rate at worst. Anecdotal: I was in my 3rd of 4th day of symptoms before I finally came up positive on antigen (at home and at testing sites; the test appeared to be the exact same). PCR showed it from day 0 but the results lag quite a bit especially right now in the US (it took 7-10 days for me to get the PCR back in early Jan).

pyk 5y ago
Best I’ve heard it described is that Antigen tests are best for detecting high virulence on days 3-5 (e.g., when the spread is happening from tons of viral production, including in the nasal passages). They are more for public health usage to prevent spread rather than diagnostic like PCR.
- mattacular 5y ago
  That aligns with my experience, that if you test positive on antigen you are definitely contagious but if you test negative you may still have the virus present in your system. PCR amplifies it so it can detect even small viral load. If only we had some guidance and solid communication from public health experts that you could conceivably trust..
- nomel 5y ago
  > They are more for public health usage to prevent spread rather than diagnostic like PCR. Like the grandparent, I know several people that have all of the covid symptoms, but tested negative early on. These people just go about their day because they think they have some other cold, then stop testing until their symptoms go away. I would probably do the same.
  - mattacular 5y ago
    My thoughts exactly. I believe the widespread misunderstanding of how to use the different tests and interpret the results will end up hurting infection rates overall. For exactly the reason you mention - it lulls people into a false sense of security. I disagree with this person's take 100% especially since PCR testing has been all but abandoned in the face of the magnitude the omicron wave.
    - pyk 5y ago
      Until a PCR can be done within 15 min instead of 2-3 day turnaround, there will always be a gap in using PCR for public health purposes. That time may come in the future for faster PCR, but it is simply not the reality yet today and we have to rely on antigen tests to at the very least rely on detecting the most virulent days to prevent super spreader events.
js2 5y ago
In Wake County, NC, rapid high-sensitivity PCR tests are available at about a half-dozen sites with results in 12 hours. Appts are have been available within 24-48 hours depending on demand. You drive into a parking lot, they hand you a test, you self-swab and give it back to them. There is other testing available - this is just the county-provided testing. It’s well run.
criddell 5y ago
If you search for PCR tests for travel, you can probably find same day tests. The place I use in Austin is $85 for a same day PCR test.
signaturefish 5y ago
They're not so much unreliable as ... really not very sensitive, as I understand it. Viral load in covid sufferers follows a curve with a rapid ramp up and a slower ramp down, both PCR and lateral-flow devices require a certain minimum level in order to read positive. PCR is much more sensitive (because it's an amplification technology, so can "see" a lower level of viral load) than lateral-flow devices - which means you can be "safe" on an LFD and "unsafe" on a PCR for a small amount of time on the ramp up and a fairly large amount of time on the ramp down. The level at which you're at risk of passing it to other humans is I _believe_ somewhere between the two tests, so it sounds like you maybe got unlucky with an LFD or two, but the results you report are about what I'd expect :( LFDs are decent for telling you if you're infectious _right now_ - they're much worse than PCRs for telling you if you have covid at all. Disclaimer: I'm a software engineer, do not take medical advice from me. My partner is a biochemist and I regularly chat with a couple of council covid-response officers, and the above is the explanation I've had from them when I've asked.

pyk 5y ago

Best I’ve heard it described is that Antigen tests are best for detecting high virulence on days 3-5 (e.g., when the spread is happening from tons of viral production, including in the nasal passages). They are more for public health usage to prevent spread rather than diagnostic like PCR.

mattacular 5y ago
That aligns with my experience, that if you test positive on antigen you are definitely contagious but if you test negative you may still have the virus present in your system. PCR amplifies it so it can detect even small viral load. If only we had some guidance and solid communication from public health experts that you could conceivably trust..
nomel 5y ago
> They are more for public health usage to prevent spread rather than diagnostic like PCR. Like the grandparent, I know several people that have all of the covid symptoms, but tested negative early on. These people just go about their day because they think they have some other cold, then stop testing until their symptoms go away. I would probably do the same.
- mattacular 5y ago
  My thoughts exactly. I believe the widespread misunderstanding of how to use the different tests and interpret the results will end up hurting infection rates overall. For exactly the reason you mention - it lulls people into a false sense of security. I disagree with this person's take 100% especially since PCR testing has been all but abandoned in the face of the magnitude the omicron wave.
  - pyk 5y ago
    Until a PCR can be done within 15 min instead of 2-3 day turnaround, there will always be a gap in using PCR for public health purposes. That time may come in the future for faster PCR, but it is simply not the reality yet today and we have to rely on antigen tests to at the very least rely on detecting the most virulent days to prevent super spreader events.

js2 5y ago

In Wake County, NC, rapid high-sensitivity PCR tests are available at about a half-dozen sites with results in 12 hours. Appts are have been available within 24-48 hours depending on demand. You drive into a parking lot, they hand you a test, you self-swab and give it back to them. There is other testing available - this is just the county-provided testing. It’s well run.

criddell 5y ago

If you search for PCR tests for travel, you can probably find same day tests. The place I use in Austin is $85 for a same day PCR test.

signaturefish 5y ago

They're not so much unreliable as ... really not very sensitive, as I understand it. Viral load in covid sufferers follows a curve with a rapid ramp up and a slower ramp down, both PCR and lateral-flow devices require a certain minimum level in order to read positive. PCR is much more sensitive (because it's an amplification technology, so can "see" a lower level of viral load) than lateral-flow devices - which means you can be "safe" on an LFD and "unsafe" on a PCR for a small amount of time on the ramp up and a fairly large amount of time on the ramp down. The level at which you're at risk of passing it to other humans is I _believe_ somewhere between the two tests, so it sounds like you maybe got unlucky with an LFD or two, but the results you report are about what I'd expect :( LFDs are decent for telling you if you're infectious _right now_ - they're much worse than PCRs for telling you if you have covid at all. Disclaimer: I'm a software engineer, do not take medical advice from me. My partner is a biochemist and I regularly chat with a couple of council covid-response officers, and the above is the explanation I've had from them when I've asked.