Congrats #blueanon nation.
You're going to start seeing a reduction in positive cases, because almost right on cue the WHO has issued guidance on Ct. Soon to come to the U.S. if not already here.
Instead of Ct values of 37-40 in PCR testing, we'll see Ct values of 30 . . . tops. Thus reducing the amplification of already low viral loads that in the vast majority of cases are both asymptomatic and non-infectious.
Cases are most certainly going to drop simply by this measure alone. How appropriately timed . . . should have bet on it.
There's only one state that requires Ct value reporting, Florida.
As someone who designs and does qPCR assays all the time - anything that comes up after cycle 30-32 is pretty much trash (spurious products, primer dimers, etc.). Reducing your cycles by 10 will save you ~15 mins per run and allow you to process more samples a day. Good, common sense molecular biology - especially for any diagnostics lab.
Yes. So the US using a 40-cycle PCR for the last 10 months or so seems like a poor choice then.
Another science nerd here so I'll also chime in. Look at nearly any qPCR experiment and you'll see you always run them for at least 40 cycles. It's where the signal crosses a set threshold that gives you a Ct value. And that part is a bit fuzzy too since depending where you set your threshold point, you can swing that Ct value at least 3-5 cycles.
Non-nerd here, but find this stuff fascinating. Cycles are exponential as well, correct? Like, if you get a test and your showing covid after 10 cycles you’ve probably got a good but in you. But if it’s 30-35... 40, you may have like a couple of little viral guys maybe hanging out in there?
I’d love to gain a nutshell/layman’s understanding of how this all works.
Hi nicname - sorry, I just saw this. This wikipedia page is a good place to start:
https://en.wikipedia.org/wiki/Real-time_polymerase_chain_reactionBecause you double the number of template strands every cycle of PCR, it is an exponential process - at least, in the beginning (the curve stops being exponential at a certain point as you start running out of reagents in the reaction). That's why the threshold to determine Ct is set low, you want to measure the reaction while the amplification is still exponential. And since everything is exponential, 10-fold differences in starting template concentration will be separated by ~3.32 Ct. So if you've got 100 million viral guys crossing the threshold at cycle 7, then samples with 100 viral guys will cross the threshold at ~ cycle 27. (And 10 viral guys will be detected at ~ cycle 30).