I have sort of the same question as Access and scheduling of FHIR Questionnaire resource, but I'm having difficulties seeing the picture how DiagnosticOrder or CarePlan can be of help, which are mentioned as ways to do this in the related question.
I want to support the use case that an practitioner can choose a Questionnaire to be filled in by a patient, so for a certain patient I want to create an event or resource so the patient app knows there is something to fill in. I could simply create an empty QuestionnaireResponse but I don't know if that's the correct way to do it.
I looked at the DiagnosticOrder and CarePlan resource, but can't really see how to reference a Questionnaire from the DiagnosticOrder resource, should it be done using the Item field? Also, where in DiagnosticOrder can the scheduling be realized? I can't find a field like 'dateScheduled' for example or some other field which denotes before which date the Questionnaire should be filled in. In CarePlan, there's an activity, but as I see it, that's more of a 'our next evaluation is scheduled next week' in stead of 'Please fill this in before next week'.
I was very surprised to find that DiagnosticOrder doesn't have an element to capture the timing over which the order should be fulfilled. That seems like a pretty clear omission to me, given that DiagnosticOrder covers lab, vitals, imaging, requests for assessments, etc. most of which will need to indicate at minimum an effective period for the order, and many will need to indicate timing frequency as well. Please submit a change request (Propose a change link at the bottom of each spec page).
As for identifying what questionnaire to complete, that could be handled by Observation.code, thought the granularity might not be what's needed. There should probably be a standard extension that allows you to identify the specific Questionnaire to be completed. That would be a useful change request as well.