Why is it not feasible for counsellors to design their own mental health questionnaire to assess clients for depression

/in /by

Why is it not feasible for counsellors to design their own mental health questionnaire to assess clients for depression, anxiety and grief and create scoring for each concept if working in community health settings in a rural area?
How can this situation be addressed using concepts of reliability, validity, and testing? What are some of the challenges posed with a modified questionnaire?