And remember that there is often the requirement for a "prior" interview in which participants discuss the trial, before the interview in which consent is gained. So it would be prudent to document the time and date of the prior interview as well as the date and time of the consent interview. Useful if this is documented in the participants notes/medical records. As Margit says, this would be useful but not 100% proof that it happened. I would suggest a documented Risk Assessment about the consent process. In this RA you can document the risks that you anticipate (for instance loss of ICF; etc) and what you propose to mitigate that risk. No solution is perfect, but it is good to document this process. Remember that as part of your escalation process for this incident; you should document what the route causes of this event were, and what Corrective and Preventive Action you are proposing to make sure it does not reoccur.