GCG and diabetes mellitus: Previous studies[7–11] have reported that a short history of diabetes, a high fasting serum C-peptide (CPR) level, a high stimulated CPR level at 6 min during glucagon stimulation (CPR6), and a high urinary C-peptide level at the start of treatment may predict the response to GLP-1 receptor agonist therapy in terms of reducing the blood glucose levels.