Below is a flow chart which may be useful to support diagnosis of Type 1/ Autoimmune Diabetes. This was developed by University of Edinburgh (ref: http://www.edinburghdiabetes.com). You may wish to also consider Type 1 diabetes genetic risk scoring for certain individual to increase diagnostic sensitivity, if you have access to this test.
This protocol may also be useful: https://www.edinburghdiabetes.com/s/C-peptide-protocol-Scotland-2021-v2.docx
Need more information on interpreting c-peptide and antibody results?
More information on how this tool calculates risk.