My university provides access to Turnitin for every student. I have prepared a manuscript ready for submission to a double-blind review conference for September 2023. Is it OK to use the plagiarism checker to assess the similarity index of my manuscript before submission? Would that be judged as plagiarised work since the review is double-blind?
No, it's probably not OK: the Turnitin End User Licence Agreement says
You agree to use the Site and Services only as follows:... if You are a student, only in connection with a registered class in which You are enrolled.
Is it OK to use the plagiarism checker to assess the similarity index of my manuscript before submission?
You can do whatever you want with texts you have written yourself. So yes, you may use Turnitin, no problem at all. As others have pointed out, you should know how to interpret the result, of course, and think about the difference between similarity and plagiarism.
Would that be judged as plagiarised work since the review is double-blind?
I do not get the logic behind this question. Using a tool such as Turnitin does not automatically imply that you plagiarised. And the type of the review process has nothing to do with it, either.
As per Wikipedia:
Plagiarism is the fraudulent representation of another person's ... as one's own original work.
So no, "assessing the similarity index of your manuscript" is most certainly not plagiarism.
However, according to https://en.wikipedia.org/wiki/Turnitin:
The essays submitted by students are stored in a database used to check for plagiarism
I'm not sure what exactly Turnitin does with the stored materials, e.g. who can access them. Since your work will be stored there, it can potentially violate the copyright required by your conference. I would check with the conference whether using Turnitin is allowed (and, as others already mentioned, I wouldn't use any such tool in the first place).