1. Since this is Twitter, I'm going to step outside my academic lane and pontificate about virology stuff.

Here goes:

Journal reviewers and editors should reject any paper that reports Ct values instead of cp/mL.

2. cp/mL (or copies per swab) is a scientifically meaningful measure of how much viral RNA is in your system.

In contrast, Ct value is the number of PCR cycles it takes to detect the RNA.

3. Ct value and cp/mL are related, but the relation varies from machine to machine.

To report cp/mL you have to first calibrate your machine against known samples. This gives you a curve that allows you to translate a Ct value from your machine into an interpretable cp/mL value.

4. Some labs do this but most don't. Those that don't report Ct values, which can be only loosely compared across studies. To my outsider eyes, this is crazy!

It's as if astronomers used uncalibrated cameras and reported their results in pixel values.

5. Reporting Ct values is better than reporting nothing quantitative at all, so I'm glad to see non-research labs moving in that direction. https://www.nytimes.com/2020/12/29/health/coronavirus-viral-load.html?action=click&module=Well&pgtype=Homepage&section=Health

6. But compared to cp/mL, Ct values are:

• Bad for scientific communication, since they can be only loosely be compared across studies

• Bad for public communication, since higher values mean less virus

Would be interested in the opinions of professional virologists.

(See short thread above on why reviewers should reject any paper that reports Ct instead of cp/mL)

@angie_rasmussen @DanielGriffinMD @MackayIM @igoodfel @CillianDeGascun @BeakerH @mugecevik @MarionKoopmans