Kink educator, community organizer, and activist.
Tornus
Karma: 176
Yes, accuracy in antigen tests seems to correlate very strongly with viral load (and presumably therefore with infectivity). This paper found 100% agreement with PCR for Ct 13-19.9 (massive viral load), all the way down to 8% agreement for Ct 30-35.
Ct (cycle time) measures how many amplification cycles were needed to detect nucleic acid. Lower Ct values indicate exponentially more nucleic acid than higher values, although Ct values are not standardized and can’t be directly compared between testing facilities.
Thank you for this! I have a few thoughts about antigen tests.
1: I’d recommend the BinaxNOW as the “standard” home antigen test in the US. Broadly speaking it’s better studied, more accurate, cheaper, and more widely available than the others. Regarding data...
2: I think the best current source of general data on home antigen tests is this meta analysis from September. The results from multiple papers over the last year have been pretty consistent, but this adds a little more power to the numbers. They come up with:
Overall: sensitivity 68%, specificity 99-10%. Sensitivity for symptomatic individuals: 72%. Sensitivity for asymptomatic individuals: 52%
Sensitivity for Ct < 25: 94%, Ct > 3: 30%. (I’ll be writing more about these results in a bit, but the short version is that this strongly supports the belief that test sensitivity depends strongly on viral load and will be highest during peak infectivity).
3: Two additional excellent papers are this one for subgroup analysis and this one for subgroup analysis and discussion of how user error affects accuracy.
4: Related to the above: accuracy seems strongly correlated with viral load, which strongly suggests multiple tests on the same individual at the same time would be highly correlated.
Yes, those are all excellent points.
I wrote this as a side reference for a deep dive on the BinaxNOW that’s coming shortly, and it’ll dig into the numerous, complex, and important issues affecting BinaxNOW accuracy. Short version: the accuracy varies substantially, largely based on viral load. And you’re correct that repeated tests on the same individual will be strongly correlated.
And you’ve convinced me to change the example you cite: I’d gone with the first person for narrative consistency, but I’m shifting it to prioritize technical accuracy.
Excellent question, and I think a lot of us are wishing we had more data on this—unfortunately, there is very little data so far. But here’s my take:
If you had J & J for your first shot, I think there’s enough evidence now to say it’s probably (p = 0.7?) better to get Pfizer or Moderna for your booster.
If you had Pfizer / Moderna for your first two shots, my instinct is that J & J might be the better choice, because there’s an argument from microbiology that mixing types might produce a more robust response.
If you had Pfizer / Moderna and want an mRNA shot for your booster, I don’t think it’ll make much difference which one you get: they’re very similar.
There’s an argument to be made that absent strong reasons to do otherwise, it’s best to follow standard practice (in this case, to get the same brand of booster as the original shots) simply because you’ll be in a larger, better-studied cohort.
A couple of sources, such as they are:
“But something has really become clear: The mixing really is most impactful when you have a DNA/adenovirus vaccine first followed by the mRNA vaccine,” Gandhi said. WaPo
The study’s researchers warned against using the findings to conclude that any one combination of vaccines was better. The study “was not powered or designed to compare between groups,” said Dr. Kirsten E. Lyke, a professor at the University of Maryland School of Medicine, who presented the data. NYTimes