Serum C-peptide measurement is an important indicator to judge the function of islets, because insulinogen secreted by human pancreatic β-cells is formed into insulin and C-peptide after enzymatic shearing, so insulin and C-peptide are secreted in equal proportion, which can reflect the function of islets equally. C peptide is not affected by exogenous insulin and can reflect the status of pancreatic islet function more accurately than insulin. In diabetes, fasting and one- or two-hour C-peptide levels are usually measured to reflect islet function, so that more accurate treatment plans can be made.