We would like to study the OOP expenditure of patients under going treatment for chronic diseases. Could you suggest any standard questionnaire for the same.
You may need to explore the literature to find possible gaps of what such studies have found. Try to craft your research question which is suited for your study population and feasible to conduct. Accordingly, you may find the best possible option to execute your study with a suitable study design and variables to explore. Based on your initial idea, it would be best suited as a cross-sectional study design. Try reflecting on the health policy practiced in your country to find possible barriers or factors that could influence OOP in your study population. Also, find validated scales for measures!
Do you have any specific disease in mind? It might make it simpler to plan a study around one condition, rather than chronic diseases as a whole. Also, in my opinion, you should consider adding a qualitative component to the study. Gather perspectives of the patients, their family (especially the person who pays for the treatment: parent/spouse/child), and the doctors.
Please have a look at the methods of following articles (in case you haven't already seen them):
1) Out-of-pocket expenditure and its determinants in the context of private healthcare sector expansion in sub-Saharan Africa urban cities: evidence from household survey in Ouagadougou, Burkina Faso (doi: 10.1186/s13104-016-1846-4)
2) Out of pocket expenditure among the households of a rural area in Puducherry, South India (doi: 10.4103/0976-9668.127312)
3) Pre-test of questions on health-related resource use and expenditure, using behaviour coding and cognitive interviewing techniques (doi: 10.1186/1472-6963-12-303)
4) Health service use, out-of-pocket payments and catastrophic health expenditure among older people in India: The WHO Study on global AGEing and adult health (SAGE) (doi: 10.1136/jech-2014-204960)
Additionally, I found these three interesting resources, maybe you will find them helpful too -
1) Measuring Household Out-of-Pocket Health Expenditure EVALUATION TOOLKIT: TOOL #2 by Jennifer Headley, Kendal Swanson, and Joy Noel Baumgartner (Duke SEAD: Evidence Lab, 2016)
2) Estimating out-of-pocket spending for national health accounts by Ravi P. Rannan-Eliya (WHO, 2006)
3) Validity and Comparability of Out-of-pocket Health Expenditure from Household Surveys: A review of the literature and current survey instruments by Richard Heijink, Ke Xu, Priyanka Saksena and, David Evans (WHO, 2011)