Many sponsors define enrolment of a subject into the trial as the date the potential subject signs the consent form, as without this signature no trial related screening or trial related documentation can be created. When there are 2 consent forms (as in Phase I trials) it would be the trial specific consent form signing that would denote the enrolment into a trial I would think.

Interesting that the EU guidance with the CTR (Q & A document April 2018) defines, “the date of the first visit of the first subject should be the date the first subject or his/her legally designated representative signs his/her first informed consent to participate in activities that are protocol directed interventions”. The phrase “participate in activities” means participate in the trial and so it is not much of a stretch to use a similar definition to define when enrolment of a single subject begins.