GAD1 and diabetes mellitus: For other clinical features, the strongest evidence was that in many observational studies markers of lower insulin secretion (including longer diabetes duration [or proxies such as insulin treatment], lower fasting C-peptide, lower urine C-peptide-to-creatinine ratio, and positive GAD or IA2 islet autoantibodies) were associated with lesser glycaemic response to GLP1-RA33–46.